As in the title, I spawned a pair of Nyxes (with Slat armour to test if an Engineer could repair slat armour in the field [of which he can't, yay]), an Engineer w/ Toolkit, set one of the Nyx's Health at 50% (and then, as the single reproduction, at 30% when 50% didn't break the slats or show visible damage but still suffered the issue - I thought I had just cancelled the Attributes change initially).

While I could get into the Nyx and saw Red damage-levels on all parts, I could not repair it until I got into the other Nyx and put a single .50 cal round into the pre-damaged Nyx, after which I was able to repair the damage to Orange-state.

I do not know whether or not other vehicles would suffer from this issue.