If ANY plug-in window touches the very top of the second monitor screen (on Mac), then N10 will not remember the correct window position and, on subsequent re-opens of the plug-in, will re-locate the plug-in to the top left of the second monitor screen.
Workaround: Resize or re-position plug-in window so that it’s slightly below the top of the screen.
Mac 10.13, Nuendo 10.1