I'm not sure if it is a bug or not, but today i was out killing monsters in Vorgardia, and I was using the skill Mug. In the middle of the fight, after using the skill Mug, its said

Your Filch proficiency has increased!


Shouldn't My Mug proficiency have increased instead?

Nope. A few skills have overlapping things, where mug is also partially based on your filch prof for success. It checks both skills when used, so it counts as a use for both mug and for filch.