Every deleted message in Mail.app also gets a copy in "Recovered Messages"

I don't know if others have seen this, and I have of course filed a bug report, but on the Catalina betas, across Macs, any mail message I delete gets duplicated in an On My Mac "Recovered Messages" folder--and I end up with an On My Mac Trash folder as well. They come back if I delete them. It's an IMAP Fastmail account. With something as specific as this it's hard to tell if it's a Catalina thing or just specific to my configuaration.