When creating an extra end dimension via a data pack, overworld music is playing, will end music is expected.

How to reproduce:

• Create a new world with the provided overworld-music data pack and cheats allowed
  (The data pack contains a simple new dimension, which uses the end dimension type and generator)
• Make sure you are in survival mode (/gamemode survival), so that the creative music is not playing
• Make sure you have music enabled / loud enough in the settings
• Teleport to the custom end dimension (/execute in test_pack:other_end run tp @p 0 70 0)
• Wait and notice how the overworld music starts playing instead of the expected end music

Possible cause and solution:

Looking into Minecraft#getSituationalMusic(), which seems to handle what music should be played, the end music is a special case and only plays, when the dimension is actually the end. Otherwise it will play the biome specific music or the normal overworld music, when no biome specific music is present.

Looking into the end biomes, it seems that they do not specific any biome music (as is the case with nether biomes), resulting in the overworld music playing.

A proposed solution would be to add the end music to the end biomes and therefore bind it to the biomes (as is the case with nether biomes) instead of the dimension.

I made a second data pack which overrides the end biomes and adds the end music to them and using this data pack it works as expected and the end music is played.