The screed outside the shower area has sagged

Randomly messing around and saying "oh, it's not that bad" are not helpful answers.

Did someone write that? No.

Which of the smart people here is actually asking what the reason for an "sinking of the screed" could be,

Hence the request for a photo! Until #x nobody (except the OP) knew which screed was meant, the one of the shower or the one of the whole room.

I don't have a photo handy right now

You can quickly take one with your phone

and I also don't really feel comfortable publishing one of my bathroom on the internet right now.

Because then everyone knows how an anonymous Hausbaer lives?! Well, nobody really cares. You also don't let the craftsman into the bathroom?

so that all the water which comes from showering (we don't have a glass door, meaning that it easily happens that water gets there)

Anyway: no idea how long your shower is. That would actually be interesting to know.
No idea how much flow your showerhead has, that there is so much water that accumulates - which usually happens at first - inside the shower to the point that one day it has reached the boundary of the walk-in shower so that it then runs into the room.
For us, the walk-in shower is two meters deep, and even with my husband nothing happens. He's a long-time shower user and stays in the shower with 42-degree water flow, which is not exactly little, in the shower.

Since the screed can obviously sink (for whatever reason), the question here is why your water does not drain along the sloped surface of the floor but instead collects and finds its way – wherever that may be.

That’s the first thing I would check. Because that is exactly the problem at first.

Have you ever cleaned the drain?

Have you ever cleaned the drain?

That was a sincere question and should be considered and answered just like your question There are quite a few people here interested in what might be causing your problem.

Has the joint to the wall cracked?

No, the silicone joint has not cracked.

Since the screed can of course settle (for whatever reason), the question arises here as to why your water does not drain on the inclined plane of the floor, but rather collects and finds its way - wherever.

That is the first thing I would check. Because that is exactly the problem in the first place.

Have you cleaned the drain before?

Yes, I have, but the problem only occurs if water gets into the non-shower area for some reason. If you shower so that the water stays only in the shower area, everything drains perfectly. But if you do not shower carefully and water gets outside, or water gets outside for some other reason, the slope in the non-shower area causes the water to run towards the wall. I quickly drew something to illustrate the situation.

And how long is the shower area?

.. and additionally, how long is the area to the left of the plan? ... we don’t know your bathroom. You describe in words what seems important to you at the moment, expect answers, but do not provide an overall picture.

And how long is the shower area?

about 1.30m

.. and additionally, how long is the area to the left of the plan? ... we don’t know your bathroom. You are describing with words what seems important to you right now, expecting answers, but you do not provide an overall picture.

It’s normal that I first disclose the information relevant to me. My question was "only" whether stricter measures apply in the bathroom area concerning the leveling of the screed, and whether a slope outside the shower area is considered a construction defect for which the builder can be held liable. I did not consider the requested information (photos + measurements) relevant for this but rather evaluated it as an abstract question.

I also believe to have already received my answer in post , and I thank everyone who wrote here.