Apple Mail bug causing data loss in macOS Catalina

According to Tsai, he's heard from several users that updating Mail's data store from Mojave to Catalina sometimes says that it has succeeded, when in fact on closer inspection it turns out that large numbers of messages are incomplete or missing entirely.In addition, users have reported the loss of message content when moving emails between mailboxes. From Tsai's post: Moving messages between mailboxes, both via drag-and-drop and AppleScript, can result in a blank message (only headers) on the Mac. If the message was moved to a server mailbox, other devices see the message as deleted. And eventually this syncs back to the first Mac, where the message disappears as well. Tsai warns that these issues are particularly pernicious because users may not realize anything's wrong unless they look at affected messages or mailboxes. Since the data is synced to the server, these problems can also propagate to other computers and devices, and relying on backups is difficult because Mail data is continually changing and there's no easy way to merge restored data with messages received since the last backup.

Tsai notes that Apple Support appears to be erroneously advising users that lost Mail data in Catalina can't be recovered from a Time Machine backup made using macOS Mojave. According to Tsai, this is not the case: Apple Mail's File -> Import Mailboxes... menu bar option can be used to selectively import them into Mail in Catalina as new local mailboxes.