I really wish I would have though to save the video but I didn’t so you are just going to have to trust me on this one.
I am on the PS4 by the way.
I was on the desert level with all the acid water (I don’t remember the name)
I was playing as Sunny whom is already at Elited
Another player was playing as Hyde.
The Hyde player through out a bomb and hit the monster (Kraken) and I got a shot off on the Kraken as well. When my mini nuke hit the kraken Hydes grenade skill mastery popped up for me and went from 75% to 100%.
I checked after the match and it did infact 3 star his grenades for me. I was already working on the 3 star but wasn’t even halfway there yet.
I certainly am not complaining about some free mastering of a character but I feel like such a bug should be known.