Odd Mail.app Font Issue

When I backup I quit Mail.app so I don’t get any new mail during the backup. When I relaunched Mail.app a good chunk of the messages I had received in the past that I had read as rich text showed up as plain text. Not all of them did, but about 70% of them did. Any message I have sent shows up as rich text because I never format messages with plain text. Eric explained to me that I can’t control the formatting of incoming mail, but I know I first read these email messages in rich text and now they’re plain text. Technically this isn’t at all a bug, and I even can get used to seeing email this way, but I still don’t understand what happened during that backup. There’s a menu option where I can choose what format to display the message in, but those options are all grayed out. I guess I’ll see what happens to future emails that I receive. I’ve tried restarting Mail.app but that didn’t do a bit of difference. I just don’t understand what could have changed about the messages when they weren’t even open to be opened. I wonder if somehow this could be a bug, and if so, could this bug be related to the unread issues Mary and Eric had.