Wierd, i tested it almost a week ago, and with alot of ammo, managed to damage the fuel tanks. I guessed it was just considered armoured. Would be good to look into it though, just in case. In the meantime ZAMAK takes 3 rounds to the fuel tank to go boom.

So I was messing around with all of the vehicles trying to find their individual hit boxes, and at the time, for some reason, I could find all of them except for the civilian truck and damage all but the HEMTT (I was using the sniper and thought a shot or a few at the tank would be enough)
I went back on and tried using a machine gun and it seemed to penetrate in a few rounds easier than the sniper, so the fuel tank hit box is in the right place, so my ticket is now considered invalid, I suppose, though a full magazine from the sniper still only makes it orange

Tl;dr, the hit box is in the right place, but may require too much effort to destroy

I reported it here.
http://feedback.arma3.com/view.php?id=11158
It's not only the HEMTT.

There are some techniques to prevent Fuel leak form single or even some more bullet holes(like use some kind of rubber or glue lining).And explode is even more impossible.

Mass closing ancient tickets with no activity for > 12 months; assume fixed or too trivial.

If this issue is still relevant in current dev build, please re-post.