I use Windows 11 Professional on my Intel desktop PC. MP3Tag is used by me to edit metadata for my FLAC and MP3 files. I primarily store my FLAC files on my NAS drive and play them with my Sonos players, and I store my MP3 files on my desktop PC drives for playing with the Apple Music Player. Until I updated to V3.29 all was fine and working for my use case. Aside from my normal usage, I would often test the FLAC files by playing them with Windows Media Player. After V3.29 a couple of days ago, most of the FLAC files modified by V3.29 generated an error when played by Windows Media Player. The error message was a little vague: "We can't open <filename.flac>. This may be because the file type is unsupported, the file extension is incorrect, or the file is corrupt." It can't be the first two reasons, so Media Player thinks it's corrupt. This is not a serious problem for me, because interestingly the modified file plays OK with VLC Media Player and Sonos players. Just not with Media Player, Windows Media Player Legacy, or I guess anything using the Windows player tools. Since I have a workaround it's not a big deal for me, but I thought you should know.
Similar reports suggest that a cover picture that is too large may be one of the possible causes.
Example:
Could you please check the size of the cover art embedded in your FLAC files?
I will check that, although I'm not sure how to effectively check the artwork size embedded in the file; I know how to check the actual artwork file, but not its size after embedded. I do tend to use large artwork files.
Interestingly, as the example you provided shows, I also experienced some random instances of the metadata not showing in the File Explore track listings.
As a side note, Windows Media Player seems to play the resulting MP3 files OK, just not the FLAC files.
The format MP3 seems not to have the same limit for embedded cover artwork.
You can view a selected music file in Mp3tag's lower left cover art panel and the size displayed like this:
If the size is ~16MB this could be the cause.
You can easily test it:
Reduce the resolution of such a big cover art in a sample music file to a lower value, save the change and see if it plays fine in Windows Media Player.
I will just use one example of a FLAC file that does not play in Windows Media Player: the embedded artwork is 3MB.
3MB might still be too large for Windows Explorer and Media Player. Try something smaller and use Utils → Optimize FLAC from the right-click context menu afterwards.
I failed to note that I actually had two artwork files embedded (cover and track listing), and the total was 5MB. So, I reduced the file size using Irfanview to a total of about 170kb, and did the Optimize FLAC operation. Now it plays in Windows Media Player and also shows the metadata in File Explorer. Strange that it plays OK with other media players even with the bigger artwork, though. I wonder what the "sweet spot" is for artwork file size?