The Bug

Additional/custom jukebox songs in data pack are ignored when uploading a data pack to a Realm in Java Edition 1.21. This causes the jukebox song to no longer be recognized as valid for the minecraft:jukebox_playable item data component or removing existing jukebox_playable item data components from non-music disc items with pre-existing jukebox_playable components on it.

How to Reproduce

1. Download a set of data and resource packs that add additional jukebox songs and playable items to the game, for example the attached data and resource packs
2. Add this data pack to your Realm in 1.21
3. In-game, attempt to apply the new jukebox song(s) to any item using the /data command to modify its minecraft:jukebox_playable item data components, or if using the example packs, use the /function command to attempt to give yourself the item with the proper components. 
4. Notice that the added jukebox song is invalid (and that this makes the only function in the example datapack invalid)  
5. Now try to add the same data pack to a singleplayer world in 1.21
6. Notice that the added jukebox song can be applied to the jukebox_playable component and dependent commands (like the function in the example pack provided) are valid and perform as intended