Thank you very much, Braza! The thread you linked seems to be the solution. Actually I'm kind of embarrassed now, because on this example project (in which I tried&succeeded to get rid of the wall cut lines in plan view in the local library "multifunctional" door we'll use in the project) I used non-composite slabs, and in our actual project we have the finish as a composite and properly set as finish...
So if I'd done this in the main project, this problem would've probably not even showed up... But at least I got to learn something new about Archicad.
I'm not at my workstation so I can't confirm if it works but I'll update when I get the chance to test this. (→ Tested this today and it's working.)
Expanding on this topic - is this behaviour documented somewhere? I mean it seems perfectly logical that doors would act like this when they're connected to floor finishes, but I don't remember seeing this discussed anywhere in the reference guide.
Edit: D'oh! The reference guide page is there in the thread, plain to see..