UE-206910 - appears to be a bug in RebuildDisplayMetrics used by FPointerEvent FSlateApplication::TransformPointerEvent(const FPointerEvent& PointerEvent, const TSharedPtr& Window) const
DisplaySize in the calculations is using the Primary monitor resolution, instead of the monitor that the cursor event is coming from, this results in bad scaling. I will try a fix on my side by multiplying mouse co-ordinates by the primary display size, and then dividing by the correct monitor resolution