Changing field settings erases saved permissions

I created a text list field for comments and set field visibility to custom. I then went ahead and set permissions and tested them. Everything worked properly. I had originally set the field to use radio boxes but I didn't like how it looked so changed only the widget to a select box. This wiped out the permissions that I had set or at least I thought it had.

When I tried to reproduce, I found that it wasn't the widget change. It happened when I edited the main field page. I had to do this to update the default value because when I changed the widget that value got erased. After updating the default value and saving this change, the permissions got erased. That clearly shouldn't happen.