When updating multiple files at the same time with tag-tag, the file modified timestamp was not updated. The particular field I was changing was Title. The problem does NOT happen when a single Title field is manually edited. However, I now tried this again with a simpler case of tag-tag (so as to avoid messing up tons of files), and the modified time WAS updated. The only thing that may have caused this was that I toggled the "preserve file modification time..." checkbox from off - to checked - to back to off. I am not sure what the default is, but if it is checked then I previously unchecked it between restarts of the program. So, this could be an issue of initialization. This was for mp3 files.
However, in doing that I noticed a separate bug. Namely, when I performed an undo, the date stamp was not changed back to the original. It would be useful if it were.