Seems I found solution, for Widget 3D Component if Space is set for World then Receive Hardware Input must be unchecked - without that combobox dropdown menu is misplaced, very weird behaviour…
I had the same issue after animating a widget from off-screen to on-screen. The widget was placed off-screen when it was loaded.
I changed the whole thing back to on-screen and animated it off-screen at construct. Now when the widget is animated back to on-screen, the drop down is at the right place. Strange, but it works (at least in 4.27)