Installation of additional insulation for underfloor heating - cables lie on the raw floor slab

Normally, there is already a bitumen strip under the interior walls. The bitumen membrane of the living areas is then connected there. But if you lay this membrane over the cables, there will be gaps at the transition of the cables from the floor to the wall, right?! I'm not an expert, but I would think it makes more sense/more logical to place it under the cables. The question is, what does the standard say about this?

Google DIN 18195 Part 4

If you lay this welding membrane over the cables, there will be gaps at the transition of the cables from the floor to the wall, won't there?!

What do you mean by that? The cables are located about 20-30cm away from all walls. Under the walls on the ground floor there is also a bitumen membrane everywhere. The foil we want to use has the advantage that we don’t have to weld it because it is self-adhesive at the edges. The foil is then glued onto the membrane under the masonry. Only at the cut points, not. If I then have to weld the foil there with the existing bitumen membrane under the masonry, the advantage of the self-adhesive feature is no longer that great.

I looked at your photo again. The cables go into the wall, so a tight sealing of the moisture barrier at the edges is not possible at all. I’ll say clearly what I think. That was a really stupid idea, wanting to do the floor insulation yourself to save 1000 euros, without the proper expertise. The craftsmen notice that and let you run around like crazy. The electrician sticks his cables onto the concrete, without saying that the waterproofing membrane is missing. He doesn’t care, it’s not his house. The bricklayer says nothing either, no hint, Mr. xy, now the waterproofing membrane has to go in. You unsuspecting… oh what, me? I’m supposed to do that? But the cables… the craftsmen laugh to themselves, the foolish client wants to do it himself, we are too expensive for him after all… The fact is, the membrane has to go under the cables.

Of course, there is a distance between the cables and the wall, but at some point, the cables have to be laid towards the light switch or socket in the wall. That means the cables go from the floor onto the wall...

So, the "structure" looks exactly like ours.

Is a COMPLETE sealing of the base slab UNDER the cables ABSOLUTELY necessary?
The walls are standing on sealing membranes according to the photo.
Just like it was done at our place.
We also don't have a complete sealing of the base slab under the cables and lines and no craftsman said anything about it.
And we all get along perfectly with them, where for sure someone would have pointed out a mistake in our case.

Maybe the whole topic also has something to do with the structure of the base slab?

So, the "structure" looks exactly like ours.

Is a COMPLETE sealing of the base slab UNDER the cables ABSOLUTELY necessary?

In my opinion, YES...
Either with bitumen membrane or Katja....