So, yesterday morning the boss of this company was here and looked at it together with me. He was friendly and explained his point of view to me. According to DIN standard, the bitumen membrane only needs to overlap, according to him. The longitudinal side is glued, only the end joints are not glued but just overlap. He says that the weight of the screed presses the membranes together so that nothing happens there. At the walls, the membranes are laid in a trough shape upwards, overlapping with the wall barrier. From his perspective, everything is fine.
The site manager also thinks it is okay. Yesterday the underfloor heating was already installed, so now nothing can be changed easily. Gluing it yourself is not possible with this tight schedule.
So...I think nothing will happen. The longitudinal sides are glued, and there shouldn’t be that many end joints anyway. Still, from my layman’s perspective, it’s not quite what is stated in the scope of work.