One solution would be to add a check box to the bottom of the dialogue to the effect that changes should be added to the tags rather than replacing the tags.

I was just playing in MMW and noticed that. One peculiarity though...if one track has a value in a field and all other selected tracks are empty, the field is populated with that entry. If there is data in more than one track, then the field is empty. If there is no data in any track, the field is also empty. In MMW, the way to deal with this is to select tracks and drag them onto the genre you want to add. There is no way to do this in MMA, but I'm assuming the goal isn't to add all MMW functionality into MMA so this is probably reasonable.

The only thing that would be nice is to have some kind of notification that entering data into a field that is populated by at least one track will erase data unexpectedly. Perhaps just a font color or cell background change from default. I can deal with not being able to change things in large numbers, but it would be nice to know I won't be screwing things up at least.

That's what is expected, it works the same way in MMW.

This might not be a bug so much as an issue that needs to be addressed (hopefully) in the future. Say you want to add a genre to a bunch of tracks. To do that, you long hold a track in your list and select it and all others to be edited. Select 3 dots and "properties" to bring up the properties editor dialog. Enter the genre and select ok. All tracks then are set to that genre. Great, right? NO! What if you have a genre already on one or more of those files? That data will be wiped and replaced by the new genre. What if I just wanted to add and not replace?

Examples:
Metal is a form of Rock, so I might want both on a track
I use a genre to tag files for certain errors (e.g., no artwork, wrong title, bad quality, etc). I can have a genre on several songs that all have no album art that I would want to add genre "fix no art" to without wiping out the original genre.

I could edit genre for each file one at a time, but it would be really nice to be able to add and not replace metadata.