Catalina Mail: Moved or Deleted Messages reappear in /On My Mac/"Recovered Messages (iCloud)"

This morning there were (18) duplicates of already received messages in the /On My Mac/"Recovered Messages (iCloud)" mailbox.I managed to clear this mailbox by moving messages into /iCloud/mailboxes or deleting them. There was a recurring anomaly when trying to delete an individual message, a dialog box would appear asking me if I was sure I wanted to delete [hundreds] of messages:

Pretty scary. Which 500 messages? Where are they? I would cancel and move the message somewhere else. I finally got rid of all the recovered messages.

Two hours later, they are back — with a vengeance. Their number has increased from (18) to (148). As soon as I had moved all of them into /iCloud/ folders. Then the Activity window started moving messages:

The numbers don’t match. What is going on and what should I do about it? Help!