Continuing to run tests on my local machine, I had just invaded the Alien Trader home world and took it home. I then blew 100% nav haz in a few of the Alien Traders space as a lot of them were still in low level ships such as a merf or scout. I made sure to leave a fig so I know if they ran into those haz.

Well, I'll let the following screen cap explain things...(Running V 3.13)

Sector  : 1986 in Alien Traders Space.
NavHaz  : 100% (Space Debris/Asteroids)

Sector  : 6259 in Alien Traders Space.
Ports   : Amphojel Primus, Class 4 (SSB)

Sector  : 6426 in Alien Traders Space.
Ports   : New Strange, Class 3 (SBB)
Fighters: 1 (belong to your Corp) [Defensive]
NavHaz  : 100% (Space Debris/Asteroids)

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 6 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Warps to Sector(s) :  1986 - 6259 - 6426

Here, I tried to ping the alien so he'd go into the 100% haz in 1986 - which he did. 
Yet, he didn't blow up.

Command [TL=00:00:00]:[3533] (?=Help)? : A
<Attack>
Attack Tavo Keithib's Scout Marauder (42,221-6) (Y/N) [N]? Yes
Combat scanners show enemy shields at 64%
How many fighters do you wish to use (0 to 10,000) [0]? 6
Your fighters encounter a powerful force-shield around the enemy ship!

You lost 6 fighter(s), 42,215 remain.
Tavo Keithib warps out of the sector!
You destroyed 0 enemy fighters, 6 remain.

Command [TL=00:00:00]:[3533] (?=Help)? : S

Long Range Scan
Select (H)olo Scan or (D)ensity Scan or (Q)uit? [D] H
One turn deducted, 267 turns left.

Sector  : 1986 in Alien Traders Space.
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 6 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
NavHaz  : 100% (Space Debris/Asteroids)

Sector  : 6259 in Alien Traders Space.
Ports   : Amphojel Primus, Class 4 (SSB)

Sector  : 6426 in Alien Traders Space.
Ports   : New Strange, Class 3 (SBB)
Fighters: 1 (belong to your Corp) [Defensive]
NavHaz  : 100% (Space Debris/Asteroids)

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Warps to Sector(s) :  1986 - 6259 - 6426

Then somehow he made his way to 6426, which is also 100% haz. He then went back to 1986.

Deployed Fighters Report Sector 6426: Alien Trader Tavo Keithib's Scout Marauder entered sector.

Command [TL=00:00:00]:[3533] (?=Help)? : S

Long Range Scan
Select (H)olo Scan or (D)ensity Scan or (Q)uit? [D] H
One turn deducted, 266 turns left.

Sector  : 1986 in Alien Traders Space.
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 6 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
NavHaz  : 100% (Space Debris/Asteroids)

Sector  : 6259 in Alien Traders Space.
Ports   : Amphojel Primus, Class 4 (SSB)

Sector  : 6426 in Alien Traders Space.
Ports   : New Strange, Class 3 (SBB)
Fighters: 1 (belong to your Corp) [Defensive] What happened to the 100% haz that was here?
If he went to 6426 as I saw on my fig report, he should have collided with the haz and blown up.

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Warps to Sector(s) :  1986 - 6259 - 6426

Tavo Keithib warps into the sector.
an Alien Trader ship is powering up weapons systems!
an Alien Trader ship launches a wave of fighters at BlueBot
Shipboard Computers Alien Trader Tavo Keithib destroyed 6 fighters.

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Warps to Sector(s) :  1986 - 6259 - 6426

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Warps to Sector(s) :  1986 - 6259 - 6426

Command [TL=00:00:00]:[3533] (?=Help)? : F
<Drop/Take Fighters>

You have 42,209 fighters available.
Your ship can support up to 65,000 Fighters, leaving 0
How many fighters do you want defending this sector?  1
Should these be (C)orporate fighters or (P)ersonal fighters? C
Should they be (D)efensive, (O)ffensive or Charge a (T)oll ? D
Done. You have 42,208 fighter(s) in close support.

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Fighters: 1 (belong to your Corp) [Defensive]
Warps to Sector(s) :  1986 - 6259 - 6426

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Fighters: 1 (belong to your Corp) [Defensive]
Warps to Sector(s) :  1986 - 6259 - 6426

Command [TL=00:00:00]:[3533] (?=Help)? : S

Long Range Scan
Select (H)olo Scan or (D)ensity Scan or (Q)uit? [D] H
One turn deducted, 265 turns left.

Sector  : 1986 in Alien Traders Space.
NavHaz  : 100% (Space Debris/Asteroids)

Sector  : 6259 in Alien Traders Space.
Ports   : Amphojel Primus, Class 4 (SSB)

Sector  : 6426 in Alien Traders Space.
Ports   : New Strange, Class 3 (SBB)
Fighters: 1 (belong to your Corp) [Defensive]

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Fighters: 1 (belong to your Corp) [Defensive]
Warps to Sector(s) :  1986 - 6259 - 6426

Tavo Keithib warps out of the sector.
Deployed Fighters Report Sector 6426: Alien Trader Tavo Keithib's Scout Marauder entered sector.
Tavo Keithib warps into the sector.

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Fighters: 1 (belong to your Corp) [Defensive]
Warps to Sector(s) :  1986 - 6259 - 6426

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Alien Tr: Menace 2nd Class Tavo Keithib, w/ 0 ftrs,
           in Amiab Rogusec (Sovremenny Scout Marauder)
Fighters: 1 (belong to your Corp) [Defensive]
Warps to Sector(s) :  1986 - 6259 - 6426

Finally I killed him so he'd stop spamming my messages with the endless fig hits.

Command [TL=00:00:00]:[3533] (?=Help)? : A
<Attack>
Attack Tavo Keithib's Scout Marauder (42,208-0) (Y/N) [N]? Yes
Combat scanners show enemy shields at 56%
How many fighters do you wish to use (0 to 10,000) [0]? 100
Your fighters encounter a powerful force-shield around the enemy ship!

For attacking this slimeball you receive 3 experience point(s).
You lost 53 fighter(s), 42,155 remain.
For defeating this villain you receive 6 experience point(s).
and your alignment went up by 41 point(s).
Excellent, you have obliterated the target!
...In fact, TOO excellent! You can't salvage anything from it!

Command [TL=00:00:00]:[3533] (?=Help)? : D
<Re-Display>

Sector  : 3533 in Alien Traders Space.
Ports   : Cygnus Primus, Class 4 (SSB)
Fighters: 1 (belong to your Corp) [Defensive]
NavHaz  : 1% (Space Debris/Asteroids)
Warps to Sector(s) :  1986 - 6259 - 6426

Command [TL=00:00:00]:[3533] (?=Help)? :

What do y'all make of this?

Yeh, this is known alien behavior. Probably a bug.

K..as long as it's known..no biggie then.

Actually a few versions back JP made an edit to the game that stops aliens from blowing up on nav haz. They will still blow up on mines and cannons, but nav haz is cleared any time it is in thier space and they enter the sector.

TWv3 Changes:
Changes for revision 48:
Change: NavHaz in alien homespace continues to be a problem. Now, when an alien detects NavHaz in a homespace sector, it will report this to the proper authorities and have it cleared away immediately.

http://www.eisonline.com/twgs/beta/

Outside of alien space navhaz affect aliens normally. I've used this before to capture powerful alien ships for cheap. Pin one in a deadend or in a one-way warp with an adjacent single fig and blow a few planets in the adjacent sector and then wait for the navhaz to wear them down. You'll want to pop some new planets to clear up the navhaz before attemping to capture the ship. Most effective in a low turn game.

I wonder if it might be a slight improvement to have the first alien into the sector actually take damage, but THEN have it cleared out. Still better than having the entire homespace filled with NavHaz, but without them being completely immune to it.

I like that idea. If the aliens want the Nav Haz cleared, let them sacrifice one of their own to do it - lol.