Fegent45,
Thanks for posting who your provider is so we could help you locate more information. We were able to do a quick search and locate some information from EE directly. You can find more information about Apple Music as a Smart Benefit here: Smart Benefits on EE. There are some FAQs on that page that answer your questions.
Regarding your question pertaining to missing music, "Need help?" at Turn on Sync Library with Apple Music has some information that can shed some light. Specifically, "If you sign out of your Apple ID or cancel your Apple Music or iTunes Match subscription, you won’t be able to access and sync your music library on all of your devices. The original song files will remain on the device that they were uploaded from." It's important to note, that if you turn off Sync Library, then change your subscription, any songs that were downloaded for offline listening will need to be downloaded again.
We wish you the best!