Newly built semi-detached house - party wall basement damp

Hello dear forum,

we need your advice:

We are currently building a semi-detached house, with the basement made of WU concrete.
Yesterday the construction staircase was installed, so we were able to enter the basement for the first time since the basement ceiling was completed. We noticed that...

[*]the joint between the basement floor and basement walls at the corner of the party wall to the exterior wall is damp
[*]and specifically only on the wall towards the other semi-detached house (see picture, left is the dry exterior wall, right is the damp wall towards the other semi-detached house).
[*]
[*]the further you move away from the exterior wall, the drier it gets. After about 2 meters it is completely dry.

We know that a sealing tape was embedded on the outside, but NOT on the interior walls between the semi-detached houses. Therefore, we strongly suspect that somehow water is getting between the two semi-detached houses. The roof has been on for about 1 month, since then no rain can reach the intermediate insulation from above (both interior walls are separated by about 4 cm thick mineral fiber insulation). However, the exterior walls are not unprotected.

Before we contact the construction manager, we wanted to get your opinion:
- do you have any ideas about the cause?
- is the observed moisture possibly normal considering the construction progress (see above)?
- or is this a defect?

Many thanks in advance.

Worst case scenario. Clearly a serious defect. Definitely construction stop until this is resolved.

Are both halves of the basement on a common base slab, or have two separate base slabs been cast together?

How should the sealing of the basement be done at all: laterally: mineral / bitumen sealing coat / slurry? But what has been done from below? WU concrete alone is not 100% waterproof, only "impermeable". That means it only prevents water from passing through if it is capped dry underneath and at least 20 cm thick. But if, for example, water stands under the basement on the concrete, then according to DIN it does not have to be waterproof, only less permeable than normal concrete.