Noise problems soundproofing new build semi-detached house?

Here are two more pictures of the expansion joint.

The joint is 1.4 mm wide,

Hm that looks like bubble wrap with a lot of air around it. No idea if that is professional.

It can also be simple things like mortar residues in the very narrow joint. This would acoustically couple both semi-detached houses. Presumably, a specialist can find this out with some effort.

I am a layman regarding semi-detached houses, but this looks to me like a serious defect ops:

Looks like the edge strip on the screed?

gross defect

A defect is a deviation from the target condition. As long as the target condition is not known, it is pointless to interpret a defect.
Therefore, should first find out what is initially owed by the landlord regarding soundproofing. Sources for this would be the rental agreement, exposé, construction descriptions, etc.

Just out of interest: what could be wrong with the floor slab that causes this problem???

The separating joint, which is executed in the clinker, must be continuous from bottom (floor slab, foundations) to top (roof structure).