I just found a particular Reddit post... apologies, think I've just regurgitated information. 
I'll link it for anyone else wanting to try animated artwork or are having the same issues in case it doesn't get completely solved here:
Tutorial: Animated Artwork for Local Library on Apple Music/iTunes : r/AppleMusic
Sadly, I've just tried it on an M4A file with no other metadata but a filename and ITUNESALBUMID, uploaded via iTunes on Windows and it's worked with no issues on my iPhone. Maybe it's the fact they're matched rather than local files that's messing with things?
The link @ohrenkino supplied up top does show more topics with a bunch of other users having similar issues with iTunes Match, it leads to this by @mplacy :
Apple Music/iTunes has gotten stupidly bad for only scanning certain metadata into its database on first import, then ignoring any changes unless it can be input via the software itself. A complete “refresh” could do it.