When I’ve added new tracks that I’ve created to the Apple Music library, it doesn’t upload to the cloud immediately.
And the more I click around and try to force it to upload, the more it refuses to do so.
But … if I ignore it and go to another app for a bit, it decides that it can now upload stuff in the background.
So - moral of the story - if you want Apple Music to upload a new track to iCloud, just go to a different app and let it do its think in the background.