I know, I says to myself, I'll update my Mac to Catalina. It's been out for a while, and lots of bugs likely fixed by now.
Mail.app is a shitshow. Data loss with empty message bodies. I've never been more annoyed at an OS update in my life. In a nutshell, I've experienced what is described here. Perfect storm of Microsoft Exchange and a local Mail rule to move messages to a local folder.
The "fix" is to move away from local folders and rules. Instead, create a rule on the Exchange server to move the email to a folder before Mail.app downloads anything. If I hadn't stumbled across that, I would have
thrown my laptop in the ocean restored Mojave from backup.
So there's two hours of my life I'm never getting back. Thanks Apple for the stellar quality control.