If the file format is FLAC that is a known problem in MC12. The FLAC decoder plugin writes the "year" values in the "YYYY-01-01" format. The "-01-01" part does not show up inside MC12, but most other programs do not interpret the tag as a plain year value. The problem has been fixed in MC13.
EDIT
If you want to preserve the existing year values, but do other file tag changes with MC12 (like adding cover art) you can disable tag writing in the "Date" field's options (Options > Library & Folders > Standard Library Fields > Date > Edit This Field > untick: Store in file tags (when possible).
You can also disable tag writing globally (Options > General > Update tags when file info changes), but then you will not be able to edit or add any file tags.