While in 21w39a if you go into the nether you'll notice in the F3 menu that you may be in a certain biome (warped/crimson forest, basalt flats, etc) but if you look around you, you'll notice ONLY netherrack, blackstone, and/or basalt. no warped blocks (minus MAYBE the vines), no crimson shrooms (minus the vines), no soul sand or even soul soil (even if you're in the soul sand valley, even then you'll just see random bone blocks). the colour of the fog will change, and the F3 menu displays the appropriate biome, but no blocks of that biome will spawn (with the exception of the basalt flats, idk why, but only that biome generates normally). Mods will spawn like normal as if they're truly in that biome, just no blocks.