Sent Folder Moved, Cannot Move Back

The Sent folder used to be just below Inbox, but I must have accidentally moved it, and it is now under “More” folders option at the bottom. When I click and drag to try to move it back, it says that Sent cannot be moved, as it is a special folder.

Is this a bug, that it can be moved but not moved back? Is there any way to restore it back to its original place?

Somehow you must have previously chosen to hide it, that was why it was in More. Right-click on the Sent folder and choose Show. That will put it back where it belongs.