This started with the prior 1.18 pre-release & this continues into PR5. It effects old & new maps.

I went to Create New World / More World Options / World Type Single Biome / Crimson Forest & what I'm getting is very cool, but it's not a crimson forest. It's effect on the Basalt Deltas is really awesome. I loaded an old map with a crimson forest & it created a sharp boarder.

On an old map, no crimson forest would generate beyond the boarder originally created. On the new map, no crimson forest generated at all.

 It seems that in the Nether & the End, no Nylium, Netherrack, Nether Wart/block, vines, sprouts, fungus, soul sand/soil, shroomlight, chorus flower/plant, or end stone generate.  Everything is covered in grass & water.