There is more than likely some cross editing taking place here. Unless you revert while editing, Yate will never restore an earlier value. Editing any metadata field in iTunes will (or should) cause the associated file to be rewritten. When they rewrite the file, they do whatever they want.

I'm assuming that the files are mp3 files. mp3 files can and frequently do contain more than one comment. Most audio based applications have the concept of a 'default' comment. What you see on the Info panel is what Yate assumes is the default comment. You can see all comments on the Comments panel. If you look at the Comments panel you may see that you have more than one comment....you may see your missing comment.

Here's the deal. Yate, out of the box treats the first comment in the file which has an empty description field as the default comment. This is the convention used by almost all audio applications. iTunes, seems to extend the requirement that the language code associated with the comment is 'eng'. Yate has a setting to force usage of this stricter 'default' determination. This option can be found in Preferences-iTunes and is called "Use strict iTunes definition of 'default' comment". I don't know how the Serato treats comments but I would assume that it is using Yate's out of the box interpretation.

Also, I don't know if you changed the default language code in Preferences-Audio. Many applications assume that the language code is 'eng'....which is Yate's default value.

Things to try:

1) look at the Comments panel. Verify that the description field is empty for the 'default' comment.
2) set Preferences-iTunes "Use strict iTunes definition of 'default'. This can only help iTunes/Yate interoperability.
3) check what your default language code is in Preferences-Audio. More than likely you should leave this as 'eng'
4) the next time you see this happen open the files in Yate and check the Comments Panel to see what's there.