[SOLVED] Incomplete ArtWork Scan

[Miss Sissy Princess]

@Miss Sissy Princess 
The FLAC specification looks quite complete to my eyes.
https://xiph.org/flac/format.html
https://xiph.org/vorbis/doc/v-comment.html

FLAC codec and associated utilities from xiph.org are IME the most widely used in Linux audio precisely because of their high quality implementations, efficient algorithms and well defined specifications.

This isn't a specification; it's a vague suggestion rather than a set of requirements:

Field names

Below is a proposed, minimal list of standard field names with a description of intended use. No single or group of field names is mandatory; a comment header may contain one, all or none of the names in this list.

...

• Individual 'vendors' may use non-standard field names within reason. The proper use of comment fields should be clear through context at this point. Abuse will be discouraged.
  
  
• There is no vendor-specific prefix to 'nonstandard' field names. Vendors should make some effort to avoid arbitrarily polluting the common namespace. We will generally collect the more useful tags here to help with standardization.
  
  
• Field names are not required to be unique (occur once) within a comment header. As an example, assume a track was recorded by three well know artists; the following is permissible, and encouraged:
               ARTIST=Dizzy Gillespie
               ARTIST=Sonny Rollins
               ARTIST=Sonny Stitt  
  

That quote from the spec confirms my prior statement:  "One of the problems with FLAC is that its tagging ("FLAC tags" are also called "Vorbis comments") is so loosey-goosey, with no set of standard, minimum, or required tags."  

I spent much of my career working to, and writing, engineering specs.  Specifications have words like "shall" and "must."  They aren't proposed lists and suggestions that no one is required to follow.  They don't 'discourage abuse or arbitrarily polluting' file formats; they define file formats.

FLAC has it's own native tagging system which is identical to that of Vorbis. They are called alternately "FLAC tags" and "Vorbis comments". It is the only tagging system required and guaranteed to be supported by FLAC implementations.

Out of convenience, the reference decoder knows how to skip ID3 tags so that they don't interfere with decoding. But you should not expect any tags beside FLAC tags to be supported in applications; some implementations may not even be able to decode a FLAC file with ID3 tags.

This is nuts!  You can have a FLAC files that plays fine in the "reference decoder" and that fails to play in decoders that are written to comply with the FLAC documentation.  People can randomly shove in ID3 tags and that's fine and dandy? 

Oddly Apple still does not support FLAC in the iTunes / Apple Music eco system while most Linux audio players support a wide variety of formats including ALAC.

Some years ago I studied up on the FLAC tagging mess after running into problems with FLAC files that would work on some players and fail on others.  If I were Apple, I wouldn't want to deal with FLAC either -- even though it has slightly more efficient compression than ALAC and incorporates ED&C.

How would you suggest the OP "fix" his .m4a files?

By determining what is wrong with them and bringing them into compliance with the specifications.  I have the feeling if the problem was with his FLAC files, you wouldn't be telling him to just convert them all to .m4a ALAC files because it's hopeless to try to repair corrupted FLAC files.