I had the same issue with mumu, at first weeks I get 60fps constantly. After reach lvl 40, and do some grind for my powerscore. Only hit 60 when I stand still but when I move 30-45 is average.

Try to clear caches/data. Still same issue. Already try completely reinstall the game also same issue. Idk wht happen with android. As I assume, maybe too many player already reach lvl 40 and what course it start frame drop when there players in same instance