Like your enthusiasm, but I think this is a bug. There is no need to write an add-on to fix a bug. This feature should work reliably without add-ons. If it doesn’t, I’m sure Anki’s contributors will try to fix it.
Edit: Ah, you’re talking about adding a custom marked button somewhere on the top. My bad, go for it 
This should not happen.
@dae What do you think about pinning these special (generated) tags to the top or giving them some other form of extra treatment (maybe their own category right beneath “Tags”)?
More important though: “Untagged” should be lowercase to fit better with the other ones (e.g. marked is lowercase too).
