The simple answer is that's how the tags were created and no one got around to changing them. The long answer is to read past the word "assumption" and to think logically. Until the actual release of the story's Part 2, "APHO" was the de facto continuation. It's also less ambiguous and confusing than "Part 2". Danbooru tags do not need to match exactly at the source, just close enough to be useful for identification.
You are correct though that if "as seen in A Post-Honkai Odyssey" is showing up, it's not accurate. I've been personally troubled while trying to clean up both Honkai games' tags with people writing misleading, wrong or even incomplete information and providing lousy reasoning like what happened with Theresa Apocalypse (Lunar Vow: Crimson Love). How these tags should be written is the way I did it for Seele Vollerei (APHO).
I like to think of them as tentative names while waiting for things to unfold.