Tiler DIN slanted tile - does it have to be like this?

Hello, our tiler justifies the tile he laid on the front side of the bathtub by saying that it has to be laid crooked according to a DIN.

Has anyone ever heard anything about that?
I could understand if it could be laid on the front side so that standing water drains into the tub, but to have it slope outward so that water must run off outside at the bathtub makes no sense to me.

Does anyone have a reliable statement on this?
Thanks!

To my knowledge, there is no DIN standard that provides for this.
Also not in DIN 18157 "Execution of coverings and coatings in the thin-bed method" in parts 1 to 3.
The information sheets of the relevant professional associations "ceramic tiles and slabs" also do not provide for bathtubs and associated chamfers. Ask the tiler or the executing craftsman which DIN he means where this is stated.
What should be considered, however, when emptying (also applies to floor areas of showers) is an independent draining of water into the designated floor drain. But this, as with large kitchens, is not a mandatory requirement, but a contractual agreement.
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Regards: KlaRa

That they could be laid at the front end so that standing water drains into the tub there

Front end? Or do you mean the top side? A photo often helps!

Hello,
Here is the photo. The tile Bär is not yet installed here.
If you look at the picture, it was installed sloping down to the right.

After the photo, it is indeed a completely different situation than what I previously pictured in my "mind's eye"!
The tiler takes over the surface prepared by the previous trade after his preliminary inspection (of the substrate).
If this surface is, for example, uneven so that he would (at least theoretically) have to take it over into his trade, he must raise his concerns.
Then it is the task of the client (from the previous trade) to instruct the responsible contractor to make corrections.
If in this case the tiler takes over a defective substrate, he is liable for the error on the part of the previous trade because he inspected it and apparently accepted the substrate as sufficiently level or angular.
However, if it is not possible for the tiler, for example because the residential building is an existing structure with - let’s call it - "very special" room and wall geometry, to lay it without optical impairment (the function must always be guaranteed!), he should have the continuation of the work confirmed in writing by his client after pointing it out.
I will summarize it like this:
From a distance and without knowledge of the local situation, we cannot clearly evaluate which aspects need to be considered.
However, my note remains that no technical regulations deal with such details as might exist here!
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Regards: KlaRa

Thank you first of all for the response.

It is important to know that there is no regulation as he claims.