Randomly attached tags

I have this annoying problem that some posts I make regularly with 2 hand picked tags later show up with a specific third one. It’s not a synonym and not set on the category. Where can it come from? In the beginning I assumed it was an accidental selection since it has the same prefix as the one I added, but since I pay attention I can say that’s not it. Is there a lot or table where I can find out where they are coming from?

Is one of the tags you add in a tag group where you configured that the thirst one is required as the parent tag?

No, at least I haven’t seen that (and also they are not really related). It also does not happen on test posts. I already thought it’s someone „helpful“ adding it but it sometimes shows up surprisingly fast