I opened my world from 1.15.2 in 1.16.1 and there some strange things:

• no loot from breaking minecart in survival by hand or by cactus
• commands works but there is no feedback from them. tried to use /gamerule - no effect. when i wrote any command in command block they works fine but there still no any output - in output line of command block only symbol "-"

Also i tried to optimize world and problem have not been solved 

When i created new world in 1.16.1 everything was fine.

P.S. it is superflat world and it exist from 2012 (1.2.5 version)