When I search for "fav:A -set:A -set:b" the search just remove one of the "-set"

For exemple, you create a new account (a) and favorite 3 images, image 1, 2 and 3;
You create a "set:a" for "image 2", and a "set:b" for image 3";
You try to do a search that would only show image 1;
You use the cheatsheet to get to this search: "fav:a -set:a -set:b", this should only show image 1, right? But the results actually show "image 1" and "image 2". If you reverse the search to "fav:a -set:b -set:a", the results are "image 1" and "image 3"

Tried multiple times, same results every time.
I own both sets, and both are private

That's a problem with metatags. Only one is allowed, so you'll have to work around it by making a superset that combines them both.