Run fsck on your disk. I had the same problem, digging into it I found APFS had hopelessly corrupted my disk, its snapshots, and the time machine backups. In the process I not only lost mail and other files, but I had unexpected app crashes and casual kernel panics. Once I found the corrupted files I had to log in into single user mode to remove them as I couldn’t even as root when logged as usual. Don’t forget to delete your APFS snapshots, if you don’t the problem will quietly sneak in again. After that you’re safe to make a new snapshot and a new backup.

Why didn’t Apple just give us ZFS instead of the shitshow APFS is?

i held off upgrading to catalina for as long as possible, due to the mail bug and other issues, like the removal of itunes, which broke all my applescripts.

alas, xcode is one of my bread-and-butter apps. recent versions require catalina, so i had to switch.

due to the mail data loss bug, i bought a third-party mail client. it's good to support small developers, i guess?

What is this bug? I've been using Mail on Catalina for the last 6 months and haven't noticed any problems.

Yep. Its why I haven't upgraded yet.