This would happen only in two ways, either the user ID is unique enough that when converted without being caught as a “bad” word in the process it turns into the ABCD visual tag, or after converted, the visual tag deemed to have bad words (or look exactly like a bad word), even after checking it more than one time, resulting in the tag falling back to the ABCD format.

I just deep dived into the comments plugin’s user ID into visual ID converting algorithm behind the scenes, that’s why I’m making a post related on this topic. Also, have you ever seen actual occurrences of tags like this happening on comments of generators? Share if you have one!