Unable to reproduce, but putting it here for future reference.

I had spawned a FIA offroad truck (unarmed) long time before this happened and used pretty much all the tools on it, so maybe it had been broken somewhere along the way.

I put 2 FIA AIs into the 2 first seats with no problem, but the 3rd seat (left backseat gunner) broke any AI object I put into it.
When putting them into the seat, you would notice right away that something is wrong. The AI is not changing animation (except one had it's left lower left turned 90 degrees, auch), and the position seems random within half a meter within the area of the seat position. When I here tried to move the AI object again, the model moved opposite of how I moved the icon (I moved it 1m left, model went 1m right). Model had also detached itself from the object logic somehow, since the model didn't disapear when I deleted the object.