Hey I have the grey grass/water/leaves bug as well. I already checked the FAQ for imcompatible mods, removed any ressource packs and disabled server ressource packs. As far as I know the server does not use BClib and the better~mods, it runs Terralith/Tectonic/Incendium/Nullscape.
At first it worked with no issues even with Resource Packs installed (just some cobblemon stuff so it shouldn't touch any of the grey blocks).
After a few days most turned grey. Removing every Resource Pack and restarting only a few trees went back to looking normal.
There are alot of "biome is null" errors so that could be the reason.