Using multiple resource packs make the game's graphics look grainy

Not exactly sure what you mean, it's a little hard to tell

Look at the top of the sand blocks of the first picture, compare them to the one on the right and you will see. I will also post a picture of leaf blocks, which seem to be the most affected

I know these are old screenshots but it still happens

What is mipmap levels set to?

The default level, Is there anything that could cause a resource pack to make the Mip map levels change?

Can you check your game logs from .minecraft/logs, and attach one from when this happened (or the latest.log if it happened the last time you opened Minecraft)? Chances are that one of your resource packs has a texture in a non-standard format that causes Minecraft to disable mipmapping, causing the grainy graphics.

I have attached my latest log file from yesterday

I'm experiencing the exact same issue in 1.16.4 VANILLA.
Using server resource-packs makes all look grainy.

Solved: this is caused when a texture has a wrong pixel dimension, example: emerald texture of 15px while it is supposed to be of 16px.

Yeah I figured that out a while ago, I had a texture in that resource pack that was 16x17, so it made that glitch happen, at least people will know how to fix it now