I just uploaded several 'cover art' images for a release, one of which had a comment 530 characters long. (The other comments were all much shorter.) MusicBrainz showed them all uploading successfully, and all the images are present on archive.org, but on MusicBrainz' cover art page, the art with the long comment is completely absent (not just the image, but the entire entry) and no edit for it appears in my 'All My Edits'. I tried re-uploading the same image with the same comment by itself, and got the same result: no error message, but no new data or edit anywhere.
Trying again with a comment 254 characters long finally succeeded (though there are now three redundant copies of that image on the archive.org listing: https://archive.org/details/mbid-8e1e6521-7169-4817-8bfd-cfe5ffc3fde6 )