basically grass and snow edges are not properly using mip-mapping / anti aliasing right as the models don't render the grass edge's properly, and the blocks in the distance seems to phase in and out. This was suppose to be fixed as of 1.7x but, this bug is still appering in 1.16x.

Possible Solutions:
use bedrock's solution of anti aliasing as a secondary option instead of mip mapping
increase volume of mip mapping beyond 4 till it supports up to maximum render distance

Steps to reproduce:
set seed 2
go to 42.7 88.06 245.9 facing east
fly up and down in creative mode with default settings and watch it happen.
the grass will start showing dirt on the sides of the midland between the woods and the mountain
the snow in the distance will start waving to as well as regular blocks but especially the snow

video of this happening:
https://www.youtube.com/watch?v=O6h3Cz3-5nc

update:
occurs with tall grass, custom non 16x16 blocks, leaves
optifine mod for 1.16.5 seems to fix this by turning on mip mapping to tri liniar and turning up anti aliasing to 16. but, optifine's attempt still doesn't look as good as bedrock edition's anti aliasing without mip mapping. I suggest taking the render code from bedrock and porting it to java. or better yet just port bedrock to java