Th bug
Overworld vegetation cannot be placed on soul soil or nylium. And if the nether vegetation can be placed on soul soil and even the overworld dirt blocks, why can't the overworld vegetation do the same?
Affected blocks:
Bamboo
Flowers (Wither Rose in MC-171120)
Fern
Grass
Sweet Berry Bush
Saplings (It wouldn't make sense to be possible for them to grow into trees on nylium/soul soil though)
Sugar Cane
Linked issues
relates to 5
MC-168264
Sugar cane cannot be placed on farmland
Open
MC-168265
Bamboo cannot be placed on farmland
Open
MC-171120
Wither roses cannot be placed on any type of nylium
Open
MC-168262
Dead bushes cannot be placed on farmland
Resolved
MC-171446
Crimson and warped fungi, roots and nether sprouts cannot be placed on mycelium
Resolved
Can confirm in 21w03a.