After noticing Offering monument being erroneously labelled a Siren, I went to
https://monster-train.fandom.com/wiki/Module:Cards/Data to check stuff up and fix things in general (Examples include giving The Sentient the "Draw" category, Giving Morselmaker the "Morsel" category, and removing both the "Shard" category and "Incant: Gain Shard 1" from Tethys apparently, among maybe more.) The thing about Module:Cards/Data is that when you update that page, it automatically updates all references to that card on the entire wiki, which actually pretty convenient. However, for some reason, it does not update Category pages; (If you check the Siren Category page, Offering Monument is still there.) (The card pages themselves, strangely enough, are correctly categorized and update automatically, strangely enough.)
For the well being of the wiki, these categories need to be updated somehow. However, I have yet to find a way...
EDIT: I've checked some category pages, and it seems like they've updated. Not sure if this was manual or automatic given the state of Furnace Tap for quite some time (removed Rage, still on Rage category page for some time), but this very welcome. Hopefully this is resolved for now.