Summary
Kelp cannot be placed on submerged soul sand blocks. This is important for making bubble columns easily.
Steps to reproduce:
Create a one block wide column of flowing water (only one source block at the top)
Put a soul sand block on the ground where the water flow ends
Try and put kelp on it
Observed Results:
Kelp does not get placed
Expected results:
Kelp should get placed
Reopening the ticket as this is a valid parity issue. Although it wasn't mentioned in the changelog, the ability to place kelp on soul sand was added in 1.16.