What is the depth of the cable?
Most likely the cables are very close to the surface.
The problem is in the plaster. The layer is probably less than 2 cm, so it's translucent.
Are the cables showing up in certain places or everywhere? If not everywhere, perhaps the plaster is not evenly thick, and therefore the cable coverage is insufficient.
The problem could be the wire.
A cheap wire with a plastic or rubber sheathing can leave a trace on the wallpaper with a plaster layer of over 20 mm.
You can also check the load, maybe they are very hot.
In my experience, I had to take everything off and do it all over again. In my old building, after the cosmetic repair, the walls also showed traces. I had to rip off the wallpaper, hole and change the wiring and glue it again. The plaster layer was about 50 mm.
Perhaps the reason is the chemical composition of the plastic insulation itself and the black dye.
Option – lay deeper or use corrugated pipes for hidden laying in the chinks.
Another option is to use custom cable in white insulation or paint the walls a dark color:)