iPhone sync

It has never worked that way except for iTunes Store media purchases you have made.

Transfer iTunes Store purchases to your Mac [Mojave version] - This feature works only for media content (not apps) bought from the iTunes Store https://support.apple.com/guide/itunes/transfer-purchases-from-a-mobile-device-itns255b2767/mac

The idea seems to be that you already have on your computer, and a backup of your computer, anything that originated from non-Apple sources. They got onto the device that way in the first place. The unspoken aspect is not allowing devices to be part of pirating music from other sources.

I don't subscribe to Apple Music which it sounds like you do. I know I have seen similar topics about personal music "disappearing" when using Apple Music but I don't even use the iTunes Store or do any syncing myself, plus I use a 15 year old version of iTunes. I have requested the topic be relocated to a forum for doing things like syncing and using Apple Music and hopefully somebody can recognize your problem and know the resolution. It may have something to do with what you have selected to display in iTunes or Music. For that matter, it would probably help others to know if you are using iTunes or Music. apps on the computer.