Yeah its prob because 1.13 modded isnt fully there yet. So you wrote music packs, nice im interested in this as the survival music has always been kind of week. The creative music is better. Do you have a music packs for 1.12.2 that you have done?
Its usually a few hours, but mine has been slow as well, it went from under review to processing and its been around 17hrs so they maybe busy or have alot to review. Mine is a pack update and server files so it may take longer for approval. Give it a bit more time and see.