Almost no small mushrooms generate on mushroom islands anymore

The fix for MC-312046 (excessive swamp mushroom generation) seems to have negative side effects on the mushrooms that should be generating on mushroom islands. Since apparently they now only generate if they have any non-air blocks above, they have becomes very rare.

(I’ve attached example screenshots of the same mushroom island generated in 26.3 and 26.4-snapshot-2, respectively.)