Try doing it but it won't work unless Mojang fixes it.
Steps to Reproduce:
Place a Nylium block
Attempt to place a Wither Rose on it
Observed Results:
Wither Roses cannot be placed on Nylium.
Expected Results:
Wither Roses should be able to be placed on Nylium.
Soul Soil is tracked at MCPE-136389, so I am updating this report to only be about nylium.