I think the issue, certainly with Music, is to do with how the Music Library displays album metadata. I have a stand-alone iTunes Match subscription. If I start with an empty Music Library (with Cloud Library synchronisation turned off) and import my full music collection I see 976 albums and 12,707 songs. All my files are .MP3, and all have consistent Artist, Album, Album Artist, Composer and Song (including the text-case - "Proper English Case"). Any non-standard characters have been replaced to eliminate problems with transferring to ExFat storage media. All files have embedded .JPG artwork at 300x300 (because I export to a number of devices that won't display larger artwork files). Files are a mixture of downloads from Apple (converted to MP3), downloads from other providers, or ripped from CDs.
When I switch synchronisation on and the Cloud Music Library has updated I see over 1000 Albums and about 13,000 songs. There is no change to the files themselves in the Music folder. What I can see is that the Music Library is showing discrepancies in the metadata - in "Album" view characters in the Album or Album Artist name have changed case (i.e. from "Dark Side of the Moon" to "Dark Side Of The Moon"). So I think what happens is that the "matched" file metadata is being displayed in preference to the folder file metadata. In Song" view I can set the view to "Downloaded Data Only" and then I see exactly12,707 songs again - but unfortunately in Album view the split albums are still there. I'm waiting for a callback from Apple, but at the moment my quickest fix when I add more music is to let the synchronisation run its course, switch synchronisation off, delete the whole library (songs, not files) and then re-import from the Music folder. Not ideal, but at least it works...