Similar issue to one I have – I uploaded all my iTunes choons to the Cloud, then decided to subscribe to Apple Music. Most of the older uploads appeared in my Music Library, but quite a few remained greyed-out with a Cloud download icon. What needs to happen, apparently, is you need to connect the phone to the Mac, select the appropriate choons then sync.
Unfortunately, my Mac Mini seemed to have developed a problem and wouldn’t boot up, I had a lot going on so left it.
A mate has had a look and found it didn’t seem to have had a problem after all, so it’s back home, I’ve got to connect it all back up and go through the sync process and fingers crossed get the missing music onto my phone. Much of it I found more recent Lossless and recent remastered versions so just replaced them, but I’ve got quite a bit that there’s no replacement for. 🤞🏼