Hey there , I recently noticed that if you are upgrading your champion's mastery level in "Loot" there is no animation for it. I've done 2 mastery upgrades , one level 6 and one level 7 and none of them have animations however it displays correct (upgraded) mastery level for that champion in "Collection".
If your client is set to "Low Spec" mode then there will be no fancy animations for any sort of level up.
This includes Account Level, Rank Level, Honor Level, and Mastery Level.
It will also disable chest and capsule opening animations.
[sg-ahri-2]
I just had the same issue as well without being in "Low Spec". The champion is the correct master level but the animation didn't play in the loot area, even though I get other animations.
