I experienced an issue after enabling Apple Music, where I saw a duplication of tracks under albums. In an attempt to resolve, I removed the album and then found that on my Mac is was unable to re-download the album. It didn't appear in purchase history and it wasn't hidden. Looking up the album showed it as purchased, but there was no option to download.
I ran a Finder search for the artist/album and found a reference to it within an iTunes associated XML file. Eventually I decided to carry out an iTunes reset using the following information How to Reset iTunes Library on Mac and Windows PC. CAUTION!!! doing this will completely reset iTunes and you will most likely need to download all your content again and also re-associate your devices with, so proceed with caution.
After going through this process, the album that I had been having issues with was available to download again. FWIW prior to this I had spent months researching and reading many of the same threads as, before deciding that the best way to usually resolve an issue within MacOS is to look at resetting preferences.