Why does embedded cover art work not appear in Apple Music on iPhone 12 after transferring music files from a Windows 11 PC?

I have been transferring my music files to my iPhone 12 to play using Apple Music. These are not music purchased from Apple. Before mobile phones had a rack of CDs. Ripped these to Music Files either MP3 or M4a. I have been transferring these to my iPhone 12. Add album using Apple Music on Win 11 PC. The sync to phone using Apple Device. The music albums are mostly going across though sometimes they miss some of the tracks. But regularly the art work doesn't appear show on the phone under Apple Music app (player). Using my Music Library app on the Win 11 PC I check that the Meta Tags such as track, album etc. are all OK. I also ensure that the cover art work has been embedded (I am setting the JPG at 850 x 850 pixels). Also using M4a format as after all that is what Apple use - don't they?? So why when I know it has embedded artwork should it not appear. Any ideas. I have seen other posts about Art Work but none for going from Win 11 PC to iPhone. This is most frustrating when I used an Android Phone I didn't have any trouble like this. So appreciate some advice. Thanks

[Re-Titled by Moderator]