I have the same problem in Win10:
[media]
It's the same for me, my world is a java converted to Bedrock. And yes, it's a flat world but with a custom terrain
I have exactly the same problem. In the last update before the 1.17, the bug seemed be attenuated, but today with the 1.17, it's worst than before..
Has ti happened to you too ?
The problem is also report on this ticket:
MCPE-140428