The Bug

Additional/custom wolf variants in data pack are ignored when uploading a data pack to a Realm in Java Edition 1.21. This causes the wolf variant to no longer exist or be recognized in the game, reverts all custom wolf variants to standard "minecraft:pale" variant.

How to Reproduce

1. Download a set of data and resource packs that adds additional wolf variants, for example the attached data and resource packs
2. Add this data pack to your Realm in 1.21
3. In-game, spawn a wolf in the biome specified in its respective <namespace>/wolf_variant/<name>.json file or use the /data command to modify an existing wolfs "variant" nbt data to the custom wolf variant desired
4. Notice that the custom wolf variant is not applied to the wolf and instead the variant is set to the default "pale" wolf variant
5. Now try to add the same data pack to a singleplayer world in 1.21
6. Notice that the wolf variant appears when spawned in its assigned biome or can be achieved using the /data command as intended