So I upgraded all the things to the 20w18a snapshot for as far as I know.
When I load up the game a lot of blocks are swapped with different blocks. E.g.
How it should be
How it is

I had this before and I think it solved itself by rebuilding and recopying or something, but I did everything 100 times from the start.

I can say that when I delete the mods folder but still load with the fabric loader, the world shows up fine. When I put in the mods (that I made myself, build for the 20w18a api) which worked on 20w16a.

This is my gradle properties file:

org.gradle.jvmargs  = -Xmx1G

#Fabric properties
minecraft_version   = 20w18a
yarn_mappings       = 20w18a+build.9
loader_version      = 0.8.2+build.194

#Mod properties
mod_version         = 0.0.0
maven_group         = one.spectra.minecraft.restream
archives_base_name  = spectra-restream

#Dependencies
fabric_api_version  = 0.10.3+build.338-1.16

When I start a clean world with the fabric loader installer, it seems to be working fine too.
Anybody knows what's up here?

When you upgrade the vanilla version, you need to delete the fabric registry files in your world/data folder. It's a bug where fabric-registry-sync doesn't keep the vanilla ids the same.

Duplicate of #464

@Prospector this was indeed the problem, thanks!