So, I think the following problem regards only CC 1v1 as tier, probably doesn't sound like a big issue but still.
Since Thief is supposed to be a physical move that makes contact, Swellow using it on a Yamask should change its ability into Mummy instead of the current one, but in this battle it didn't happen.
The battle didn't show any kind of message about Mummy. Screen from the battle.
I wasn't sure about the lack of the Mummy message so, few minutes later, I did two battles against two Swellow.
The first one was against a Swellow with an item and one without it, both without Scrappy if it matters.
My Cofagrigus had Petaya Berry as the Yamask in CC 1v1.
[Mummy message was shown]
Then I repeated the test for the second time with the help of two Scrappy Swellow, one with an item and one without it.
I was using a Yamask with Petaya Berry as in the previous battles.
[Mummy message was shown]
Actually that's when you only have 1 poke left. I did a check because ingame when you kill someone with mummy, and that's the last pokemon, the message doesn't show. But i forgot something which led to that bug. Fixed.