Bitumen waterproofing membrane under the stairs is missing

Hello,
this concerns a 2-family end-of-terrace house under construction, where a concrete staircase (ground floor to upper floor) has been installed. The area under the staircase is separated from the ground floor apartment by a wall. The wall is not yet complete at the top, so it was possible to take a photo of the area under the staircase to determine whether there is water present that could have penetrated through the roof, which was not closed until recently. I could not detect any water, but I found that the contractually agreed bitumen sealing membrane is missing there. The relevant area is 12.4 meters north, 6.2 meters south, 5.3 meters east, and 2.0 meters west from the end of the floor slab. No groundwater was detected in the soil report, but rainfall-related backwater buildup is possible.
The general contractor (GU) was informed and wanted to consider whether to make corrections. I have not heard anything about it for 2 weeks.
What damages could occur if no corrections are made?
Thank you in advance for your answers.

Comprehension question:
Is the "cavity" under the stairs completely bricked up? How many residential units does the 2-family terraced house have, or how does the staircase run through the floor plan (blueprint)?

Is the (beer?) bottle lying there on the floor slab and does it stay there?
Normally, a masonry waterproofing against water from below should be applied below the first row of bricks. Is the floor slab insulated from below and on the sides?

It was my idea to wall up here for acoustic reasons, because the two apartments (1st on the ground floor, 2nd on the upper floor plus attic) are separated by the concrete staircase. The bottle was probably just left lying around before the wall was built; it would have broken if it had been thrown in. A masonry waterproofing below the first row of bricks was regularly installed, but I cannot consciously remember the area around the staircase. Insulation was applied on the side of the floor slab; unfortunately, I cannot say how it looks underneath.

Addition: The floor structure on the ground floor is intended to have a total height of 18 cm later, and on the upper floor / attic a height of 15 cm. Perhaps this answers the question of whether the floor slab was insulated from below.

phew, well I'm clueless here!

And option: Command half-back "set narrow soundproof door wall breakthrough" no longer works? You could basically create a small basement room there and if it’s still too noisy, you can always wall it up again.