We have gone through such topics one after another in Nebau and still have them. I constantly hear about a DIN or tolerances, where other craftsmen or consulted experts tell me the opposite.
That is of course very annoying and I can exactly imagine how you feel! o_O You pay properly, don’t haggle, use the local company, maybe even still have coffee, cake, etc., same here..... and then something like this and on top of that an ignorant reaction.
The situation seems quite clear to me that this is not okay; the question is rather how you yourself deal with the knowledge of your rights with someone who has a pathological defensive reflex. The usual question, then, about being right and getting justice.
Besides, I would (as we do) look at how to make it nice anyway and I think there are still ways to do that if you cannot enforce your “right” to correction/replacement. Stomach ulcers or permanent teeth grinding when entering the stairs shouldn’t be the solution.
In addition to the options mentioned, you should maybe still consider a molding, although that doesn’t have to be the ugly skirting board you might have in mind. Possibly something with metal or similar could cover the maxi joint but still look intentional. With us, they drilled through a window in the visible area to lay a cable; this is now supposed to be covered with a rail.... just to give a small example of highly praised craftsmanship.
At least you are not alone, even if unfortunately nobody fixes the joint.