Since this is no longer something for the support (has been confirmed as problem and not being technically solvable atm) this better suits here. See here, if you want
This was the answer from Mr. Chips for my problem:
Unfortunately this sort of thing is a game engine limitation more than anything; multiple layers of transparent materials are extremely challenging to handle, and neither BeamNG’s game engine nor Automation’s Unreal Engine 4.27 can handle them adequately.
The easiest way you can fix this is to use cutouts to remove the red tail light material where you are going to place your indicator fixtures.
Well, I tried it with cutouts. Helps nothing, just does not work.
But maybe the problem seats in front of the PC. Has anyone here experience with this and successfully placed indicators on top of the taillights that appeared flawlessly in BeamNG?
I might have a solution after Trial and Error for some weeks but I need to do more testing just to be sure