I’m experiencing exactly the same issue, after some time while working the right click is not possible anymore. Resizing the UE window or moving the UE window on the second monitor usually fixes the issue temporarly.
I don’t have Task manager setting with “always on top”, I dont have task manager open, and I still get the issue.
UE4.27.2, Windows 10, Nvidia GTX2070, two monitors.
This problem started for me few months ago, I didnt had it before and I’m using UE since years.