Thank you Oleg,
I have been able to get one to hit me but they never respawn, I hunted the other one for hours while the stuck one was on screen. I can't target them so I don't know if they go unreachable or not.
If they're trying to get to you but are not able to attack you, they'll be unreachable. If you break aggro, for example by moving far enough away that they go off your radar, dying, or getting in a flying vehicle, it'll reset it so it's important that you don't do that. If you go close enough that they can hit you, despite being trapped, it'll reset it. By reset I mean that they won't despawn, though they may stay unreachable if it's not due to them hitting you.
As long as you can avoid any of those things, they'll stay in an unreachable state and will eventually despawn and appear somewhere else. I don't know how long it takes, I guess 5-10 minutes, but never timed it. When they reappear they'll probably be marked unreachable still, but will revert to normal when they hit you again, as long as they haven't respawned into another trapped location.
I first identified this behaviour when I was working on the Armax mission southeast of Fort Victoria, it happens a lot there. With no range on their attack it's easy to get those into the right state. The long range on the Exterminators will make it more difficult (perhaps even impossible).
If the system can identify when a mob that's trying to hit you is trapped, surely it should be able to identify when a mob that isn't trying to hit you is trapped, so there's no reason for this to happen, it's just shoddy programming and lack of attention to detail as usual.