The Bug

It seems like, this bug was missed when world generation changed back in 1.13.

Compared to 1.12.2, mushrooms generate much less commonly in 1.13 and any version after that.

Note that there are differences in population in general (see MC-124985), which is why for example trees and lily pads won't match up

How to reproduce

1. Generate a new world with seed 91 in 1.12.2 and run /tp @s -330.81 65.75 164.31 124.50 27.15
2. Generate a world with the same seed in the current version and run /execute in minecraft:overworld run tp @s -330.81 65.75 164.31 124.50 27.15

Look around in both swamps and notice how mushrooms are way more sparse in the current version, compared to 1.12

Code analysis by Barteks2x in MC-229557:

The issue has been verified in mod dev environment by adding a conditional breakpoint on block place in RandomPatchFeature#place with condition "new ChunkPos(context.origin()).equals(new ChunkPos(mutableBlockPos)) && blockState.getBlock() == Blocks.BROWN_MUSHROOM" - the breakpoint never hits, so no mushroom blocks are ever placed in the chunk where features are actually generated.