I too have been getting this issue, and after looking into it, in my case its caused by my mouse occasionally performing a second click after releasing the button. I was able to determine this using this event logger. I would suggest removing the events mouse over, out, enter, and leave for testing. What happens is when the button is released, the mouse up event fires, shortly followed by another mouse down and mouse up. UE4 is selecting objects because the second click occurs without the mouse moving.
I was able to fix it using X-mouse button control with its ‘ignore rapid clicks’ setting.