When moving messages from the inbox to a folder, even with a group highlighted, they only move one at a time
When I move a selection of messages with drag and drop the graphic shows the whole group but only one drops from the inbox into the folder. If I do the same between folders all the messages drop into the new folder. I am using vers 68.4.2 (32-bit) I have recently upgraded from a very much older version where I had no issue
Alla svar (1)
I would expect that if the folder you are copying/moving from is in an IMAP account, as each individual message has to have the changes queued with the server and notified to the server and a final sync against the canonical source (the server copy) before the copy move occurs to a folder outside of the account.
I am assuming imap to local folders. It will eventually probably just loose mails as the queue gets corrupted by things like anti virus scanning. You should seriously consider using POP if you are routinely going to make local copies of the mail.