Shower slope in the wrong direction

The formation of fronts happens immediately because the craftsman fears extra work and wants to avoid it in principle at first.

Very human to want to avoid extra work, even if not professional. But a "front" only forms later if the request for correction is not complied with. For sayings like it was like this before, no front is made, but they are smiled away and the concrete agreement on how the trade should look is pointed out. In the case of the OP, the floor must be redone - there is no other way. If the tiler has otherwise done good work, it would be in the mutual interest that he does it. That he does not want to do it gladly - taken as given.

In the case of the OP, the floor must be redone - there is no other way. If the tiler otherwise did a good job, it would be in the mutual interest that he does it. That he does not want to do it gladly - whatever.

The tiler should have complained about the work of the previous tradesman. "Once again, a Higher Regional Court (3 U 814/14) has pointed out that every craft business is obliged to inspect the preceding trades. If defects are found, they must be reported in writing to the client."

The tiler should have complained about the work of the previous trade.

If everyone does that, it leads to a standstill. Legalism is often far from common sense. Perhaps a leveling compound would have sufficed instead of pushing the rework onto the predecessor.
That is exactly why the advice is to let the tiler solve it, if possible and if he is willing and able. If now the predecessor has to be involved, the show has to be played again, the tiler comes back and does what he could have done long ago, you may be legally on the safe side, but it is not effective at all.

Which craftsman admits mistakes? I don't know any.

That's a shame. On our construction site, an open approach to mistakes and misunderstandings was completely normal – and there will be few mistake-free construction sites.

When I asked why, I was then brazenly told,...

This is a classic in going wrong communication when it comes to mistakes. To the question "why," you almost always get an explanation – that’s clear, because people respond to questions as they are asked. Since the counterpart already knows that the result is not desired and suspects that their answer will not be satisfactory, they will look for an answer to which the questioner can poorly react or that silences them – e.g., by referring to their experience or some claim. When asking "how do we now manage to..." the likelihood increases enormously that the conversation will not be about the origin of the problem but about its solution. This, of course, doesn’t always work either, but very often, whereas the "why question" regularly fails when addressing mistakes.

When asked "how do we now manage to..." the likelihood significantly increases that the discussion focuses not on the cause of the problem, but on its solution.

I can only confirm that. In our case, the plumber had made a mistake that needed to be fixed quickly (since the plasterer was already waiting and the front wall also still had to be built). Of course, I was annoyed about the mistake at home (not in front of the installer), but I also immediately thought of a solution. The next day, I pointed out the incorrect installation to the installer and directly suggested the alternative (which he also had in mind, meaning it was an immediate confirmation for him that this solution was acceptable to me). This way, the defect was quickly and smoothly fixed, and the relationship with the installer is still good. In my opinion, maintaining a good relationship with the craftsmen is generally important. In addition to respectful behavior, an occasional snack or free drinks on the construction site really help a lot.

Of course, this is of little help to the original poster here, since the only solution in this case can be that the tiles need to be removed and the floor corrected. However, the tile setter is not necessarily to blame, and I would communicate that as well, especially since his work was good but unfortunately done on an incorrectly installed floor (at least judging by what can be seen in the picture).

The tiler should have complained about the work of the previous trade.

I agree.

If everyone does that, it leads to a standstill. Legalism is often far from common sense. Perhaps a leveling compound would have been sufficient instead of shifting the retouching onto the predecessor.

The leveling compound also has disadvantages because then the entire floor is raised and a small step at the door is created. A perfect craftsman would have addressed that in advance as well. At least to factor in the additional cost for the floor leveling.