My reports history shows that the topic ID changes after a report has been handled by a moderator (as long as the report status is "good"). I think this is because posts aren't really deleted, but are moved to a sort of trash can, which is not accessible to regular members. I guess every topic has a corresponding, "shadow" topic ID in the database, which contains deleted posts from that topic.
Yeah, that's how moderation on this forum (and many more) work. I was under the impression that this is what could be happening, but since I only had my own topic ID history, I couldn't really reach a conclussion. Thanks for the confirmation! I'll now have to keep on reporting each and every shitpost I see; not only the ones at patrol