The cutouts have proved to be useless. However, I was not willing to let go of this.
After more than 100 hours (according to the steam counter) I found the solution, and it is so damn simple that I could run against a wall for not having found it earlier.
@MrChips gave me the right idea to solve this, by admitting:
neither BeamNG’s game engine nor Automation’s Unreal Engine 4.27 can handle them adequately.
So, what did I do? All the lights that are layer 2, on top of the red material, stick out a bit. Since the hours of testing already showed me that if there is another material than taillights behind it, it would work, just as stated by MrChips. So I made sure that the layer 2 really sticks out a bit. Then I copied the fixture, changed it to the general car color, sized it to 0,9 in extent to make it hide behind the indicators and reverse lights… as you can see in the screenshot, it needs to stick out enough to be ABOVE the red layer 1 and BEHIND the yellow layer 2!
So now the game considers the last layer in front of the transparent material as the one below the indicators, and since this is now the body color, everything should work.
I tried it 5 times per vehicle and this is the result: IT WORKS!
Just to make it clear: The fixture copy of the second layer must EXACTLY fit between both layers AND must have NO TRANSPARENT MATERIAL.
It seemed to have worked flawless, I spawned all these three vehicles multiple times without serious errors.