Technical note 20200110: Apple Mail- Big Sur - Gmail accounts: Moving email results in a copy.
We have identified an issue with Apple Mail on Big Sur in relation to Gmail accounts. At present, Apple Mail, when invoked via the Scripting Bridge, will copy, rather than move, emails into Gmail Mailboxes. We have flagged this issue with Apple Development support. Note that EFA moves emails without problem from Gmail mailboxes into non-Gmail mailboxes.
Exchange and iCloud accounts are not affected by this bug.
We will provide an update once we've heard back from Apple.