Music from Apple One not in iCloud

• Do you have Sync Library turned on?

Use Sync Library with your Apple Music subscription - Apple Support

• Are you a member of a family plan and not the Organizer?

• Do you see iCloud storage being used when viewing at Settings > [name on top] > iCloud > Manage Account Storage?

It you are looking for your playlists created with a previous subscription, they are not saved. Once an Apple Music subscription is cancelled all playlists and downloaded music from the previous subscription is removed. You would need to create new playlists again and/or download music if you want them available for offline use,

Did you recently transfer from an Individual Apple Music plan to Apple One?

Not sure if the music you are referring to was when you used the service previously using a subscription and now wanting to access it again with Apple One or something else.