Just tried and could not repro this. Shot it until it was 3-4%, let a zombie hit it down to 0%, got out and unloaded on it with an M60 and it blew up. Got 100xp I think.
Tried again and got it to work. Not sure why it didn't do it the first time. BUT, I also tried it with the vanilla 4x4 and it did the same thing. So it seems to be a game bug, not my mod. I would advise you to go post it in the bug forums so TFP can take a look at it.