Thanks @Lunade Lunade! I see you deleted the Biomes folder- I suppose that means the Lush Caves are gone? Does Mojang take on volunteer bug hunters? If so, I'd be down to sign an NDA and spend a few days digging through the codebase to squash this one.
Does anyone know how to disable logging to preserve SSD health? And perhaps where to edit a Biome Id?
This makes the game unplayable in 21w16a and 21w17a.
This fix was implemented back in January for this specific bug in case this helps anyone.