When uploading a world that includes a custom data pack with custom worldgen settings, worldgen settings are removed. To reproduce this, create a world and set up a data pack with custom worldgen setting that changes the generation patterns of any dimension (I discovered this using Starmute's Incendium worldgen datapack, but others I have tried have the same issue). Then, once you verify the data pack is working, upload the data pack to realms. All new worldgen data will be removed, often generating a cliff separating the blocks already generated from the new chunks generated on the realm.