Duplicate artist that WILL NOT disappear please help!

Everything is the same on both sides, info-wise, I went through all of the fields all of the Sorting fields everything.

I even tried changing the artist name to something different, then changing it back, nothing changed.

Now the only difference I managed to find is in the general Get Info tab of the artists themselves:

The left one is "as should be", 'cause all other artists that don't have duplicates have that info.

Whereas the right one, which I think is "the duplicate", has this field that doesn't reflect the artist.

But I keep on changing it, and it goes back to Mixed...by itself! What on Earth am I doing wrong?

Is that the problem? How do I effectively CHANGE IT? Thank you!