Moving sent mail from IMAP account to "On my Mac"

I am trying to reduce the volume of sent mail in one of my email accounts. It is an iMap account. If I drag emails from the Sent folder of the IMAP account to the Sent folder "On my Mac" the emails seem to disappear from the IMAP account Sent folder. I was not expecting this.

I thought dragging would be a "copy" not a move, and that I would then have to delete them from the IMAP Sent folder. I thought that deletion from the IMAP server only happened when dragging mails from the IMAP Inbox to an "On my Mac" folder.

Does anyone with more expertise know if this is the expected behaviour with Sent mail in Mac Mail and IMAP? Or is something anomalous happening?