I'm aware of iTunes Match vs Apple Music Sync Library. Incorporating legacy services into new ones is always a product management tap dance of tradeoffs.
My point is that bring your own library of pre-ripped music into a subscription service for syncing is rare these days unless you're over a certain age or are an audiophile (at which point there are better alternatives like Roon). If you're going to do rare things, then there will always be nuances to understanding the tradeoffs Apple took to support you.
It's not rare for anyone who already has a collection of music, e.g. because they are coming from a different service. It doesn't have to be ripped - it may well have been purchased via numerous existing services that sell downloadable files, from Amazon to Apple itself.
My point is that bring your own library of pre-ripped music into a subscription service for syncing is rare these days unless you're over a certain age or are an audiophile (at which point there are better alternatives like Roon). If you're going to do rare things, then there will always be nuances to understanding the tradeoffs Apple took to support you.