I made the huge mistake of creating a room reminder every 1 minute (this is a testing room thankfully) that begins with a newline for formatting.
Evidently, those can't be cancelled. The bot just ignores the message. If I tweak the formatting at all, add whitespace etc, the text doesn't match exactly so it says "unknown reminder"
Kicking the bot from the room doesn't work - when you re-invite the reminders will come right back.