I didn't even know this wasn't multiplayer compatible as it worked the first time we played together without any issues. Now the next day it complains about mismatch between client and server. Can you maybe fix this? I'm not even sure why is there a state to check given this is just a visual thing without any gameplay impact.

Hello! I'm surprised it worked on multiplayer at all :D

This mod does actually have a state, because the visual tiles and chunk information are stored and maintained there. (There's some optimization in the back-end to make the mod performant)
As this is my first mod, I don't have much experience with developing for multiplayer. After reading the guides, I anticipated having to re-write large parts of the core logic to make it work. But from what you're saying, it might be possible to add rough support by cutting some corners.

As you can probably tell, I don't often have time to sit down on and work on this. So I won't promise anything, but maybe it will happen at some point.
Thanks for your interest anyway! :)

Totally understand, thanks for answering though! Yeah, sacrificing the caching you got going on to make it effectively entirely "client sided" and stateless would be a nice way to get multiplayer working without much pain (I'm hoping).