Why does Apple Music split up albums?

Solution 1:

There may be another cause, but here is a way worked for me:

1. In the Songs view, sort by the album or artist or anything that puts the tracks of the "two" albums together.
2. Select at least one track from each album (multiple selection).
3. Cmd-I (Get Info)
4. If there is any metadata shown faintly as "Mixed," change it to whatever it should be (other than comments, or things like track, disk, that should be different).
5. If artist is supposed to be different, don't change that. Instead, select (turn on) the compilation checkbox.
6. Click OK and see whether they have merged.

One time, I had a split album where none of the metadata were "mixed." In that case, it was fixed by changing the compilation check box, clicking OK, then opening Cmd-I again and changing it back.