In 1.21.60.27 preview, the downfall value of the swampland biome was changed to 0.9 to match Java Edition. However, the swampland_mutated biome, which is used in worlds version-locked to 1.17 or prior, is still using the incorrect 0.5 value.

From the data/definitions/biomes/swampland.biome.json file in the app package:

From the data/definitions/biomes/swampland_mutated.biome.json file in the app package:

Expected Result

Swampland and mutated swampland biomes have identical downfall values of 0.9.

Observed Result

Mutated swampland still has an outdated downfall value of 0.5.