I do not know what is happening but on one world in particular in the replay all moded blocks are replaced with bits of other moded blocks, waterlogged moded blocks, or waterlogged varmints of themselves.

This is still an issue but a fix from Reddit.

Make a new world-
Delete all files but the level.dat and level.DAT_OLD files-
copy all files except for the level.dat and level.DAT_OLD files from the old world-
play this new world and the replay should work.

NOTE: put all of your inventory in a barrel or something as you will loose everything on your person in the new world. also anything in an item frame and the frames themselves will vanish. also all entities like villagers will despawn, yes this includes pets.

ver. 1.20.1 latest release

A further note to the above fix. This absolutely jacks up your world gen. I will note that I am using tectonic, but there are strange chunk swaps and sudden mountain range fall offs so on so forth

OK the STEM cell it seems is the level.dat file of the seed of your world

essentially if you run into the problem stated above, make a new world with the same seed. Delete all files but the level.dat file from this new world. Copy all files from the old world except the level.dat file to this new world. Play and enjoy.

I have seemingly found a fix for this. here is the step by step:

go to the failing world
/seed and copy seed
create new world with that seed
play in this new world a bit, don't build just have it open.
open your world folder and quit the game
copy all the old world stuff besides level.dat and level.dat_old files
remove all the new world's files except for the level.dat and level.dat_old files
paste in the old world files to the new world
test
if all is well then congrats your world functions properly

note: you may be able to keep level.dat or level.dat_old file but I haven't run this test yet. One of the two is the culprit but only one of them.

What about multiplayer? Does the server owner have to do this or does this only happen in singleplayer?

What about multiplayer? Does the server owner have to do this or does this only happen in singleplayer?

Unknown, it may be a fix to do so but as always backup files.