It happened to me too with a super Logitech mouse that I have.
I blamed Unreal first then I tested dragging on the desktop and observed context menu appearing while dragging.
So, I suggest to test how dragging behaves on desktop screen (or somewhere outside Unreal editor), maybe the mouse is the culprit (I know it is hard to believe it could be a mouse problem, especially when you have a good gaming mouse).
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