What version of dreamweaver and what OS are you using?
This is a Dreamweaver refresh issue. It might be possible to work around by adding a second comparison row and changing focus back and forth and deleting it. You may be able to tab into the field to use it as well, but I'm not seeing the issue, so I'd have to play with it.
If you give me your machine specifications, then I might be able to reproduce it and get it fixed for you as well.