When using the addon many times you will get diffrent results for applying the same query.
For example:
mythic
mythic and tanks <= 0

This does not happen because the grps expire because after refreshing a random amount of time you will see the same groups sooner or later again.

Also it feels like the difficulty filter is not working properly.

Addon version is 2.1.1.

For future reference: also see the FAQ why it behaves like that.