Gaining mastery on a champ I don't own

CLIENT Here are the champs I own currently. https://gyazo.com/a5b5a84b6c38d2c7cbd599c2ddc2ffb6 I played Riven on free week and I gained unwanted mastery points on my profile: https://gyazo.com/4808272a24e57cb45a4b099c12afd8bf It was just one time it isn't a frequent thing, I just want to know if this can be removed. I want my profile to only have one champ on it.

Isn't there a notification that pops up after you finish a game that says you can't gain mastery for a champ you don't own? I may be blind af.

EDIT: I also played an Ahri and Miss Fortune game in normals as well and didn't get a mastery on them so why did the Riven one happen? https://gyazo.com/aa7966828dac4a658f13928a371ee439