I tried now to give a better impression of what is wrong.
By coincidence, my car spawned correct at first, so I made a screenshot.
Usually, the car spawns with indicators and/or reverse lights missing on one or both sides, like this:
I already tried tripling the fixtures, so that at least one would always spawn - does not work. If I remove the red material, it works, but well, this looks equally bad. Is there a way to solve this? Happens every time when using lights on top of another.