I think I’ve found what’s causing this phenomenon!
It seems every discussion has a numeric identifier, which appears at the end of the URL; this is the key to find the discussion in the database, and anything that goes before it is unused. This discussion has the number 47564, which is shared between “Sentence comments end up in wrong thread” and “Ob es sie gibt?”
I’m guessing that the hashing algorithm used to generate these identifiers, based on the name of the discussion, is weak, i.e., unable to come up with unique identifiers for a large number of entries, or that the key space is too small (only five decimal characters?).