If you edit the "Comment" Tag by typing text into that column, it will show in the display, will be updated in the MJ database, but will NOT be updated in the ID3v1 tag inside the file! The first clue that this is happening is that when you edit Title, Artist, etc., while the song is playing, it will restart the song, presumably because the file needs to be rewritten. However, if you edit the Comment tag while a song is playing, it keeps right on playing. This is just a CLUE on what is happening--you can confirm this by viewing the end of the file with a text editor and verifying that the comment tag didn't take. Or reimport the file and the comment tag will not reflect what you entered.
There is an easy workaround once you know this is happening (and it's a potentially dangerous bug if, like me, you store lots of custom classification information in the comment field, since you could lose it all without knowing why!): Any subsequent operation that rewrites ID3v1 tags will force the comment tag to be rewritten too. So, for instance, edit the Album Name, delete one character and retype the same character (to force a rewrite) and when the Album name is rewritten, the comment will be correctly copied from the database into the MP3 file.
If you have valuable (i.e., time-consuming to create) information in the comment field that was typed in within MJ8 (as opposed to being imported from somewhere else), then you really want to go through the steps of forcing this operation for all your files, otherwise your comments may only be in the MJ database and NOT within the MP3 file. You don't have to do it one file at a time; sort by Genre or Album Name (for instance)--something that groups large number of files under the same heading, and just do the edit operation in groups of same-Genre or same-Album, by selecting the group and retyping the same name to force the rewrite.
My tests are on MJ8--I have not checked whether this has been fixed in MC9. Also, I have not checked if the ID3v2 comment tag has the same problem (mainly since it's more difficult to find the v2 tag with a text editor), although I do have both v1 and v2 tags turned on.