Leaves are considered solid faces for block placement

Blocks that previously could not be placed on leaves, can now be placed on leaves.

Analysis (mojang mappings)

This is speculative, but it seems to be caused by the BlockState cache, which caches the results of isFaceSturdy for all blockstates shortly after blocks are defined. If this runs before blocks are added to tags, the check for the leaves tag in Block.isFaceSturdy will fail once and never be run again, since the cache will be used from that point onward.