Walls on the LOD edges, CLIENT_ONLY?
I tried to use 1.21.10 with 3.2.6b, but I had problems with not rendering LODs outside of vanilla range. On this server I found advice to try nightly build. It does look like that issue was resolved, but I sometimes have those huge walls on the edge of LOD rendering and sometimes holes while rendering. I also see that i have CLIENT_ONLY without DH full support or something like that. I am running now 3.2.7b on client and server. The only things in config I've changed are: INTERNAL_SERVER rendering, no generationBounds limit, block quality render, nsizedGeneration, upsampleLowerDetailsLods. I have the same config on client and server. I am using compatible shaders and everything is the same without them.
