MC reads and writes many (but not all) ID3v2.3 v tag frames. If the file contains some ID3v2.3 tag frames that are not recognized MC leaves them unaltered.
However, the so-called multi-line comment frames are exception. MC uses these for its custom fields and when MC updates tags it rewrites the whole comment frame block with new data from the library.
Tag&Rename uses a custom comment tag for the Catalog Number and unfortunately MC cannot preserve this. For the other mentioned tags T&R uses standard dedicated ID3v2.3 fields and MC does not alter them.
Also e.g. iTunes and Musicmatch use some comment frames that MC erases automatically on a tag update.
It would be better if MC could only add its own comments instead of replacing the comment block.