I joined a multiplayer server yesterday and as I joined, the world was covered in magenta beds. I thought that this was part of the server until I discovered that the issue was on my end only. I found out that Blockus was the mod that was causing this issue. It isnt just a rendering issue either, as I use the mod Jade and that shows that the blocks are in fact what they are shown to be. I tried updating the version of the mod however that had no effect either. I am playing on Fabric 1.21.1, version 0.16.5.

Blockus needs to be installed on both the client and the server. You either have to uninstall Blockus or install it on the server. This isn't exclusive to Blockus, many other content mods will mix up blockstates on vanilla servers.