Same here.
I use 3 Monitors 2x1080p@60hz and the main monitor 1440p@144hz
GPU RTX 2080ti
something i figured out:
When using a single insrance of UE on all 3 monitors the problem only occures on the main monitor, on the other 2 UE acts normal. Maybe it has to do witch the difference between the 60hz and the 144hz monitor.
As a workaraound i use the nvidia control panel to limit the unreal engine at 60fps, UE get´s kind of laggy using this workaraound but no more disappearing of the right-click menu.