I have the same issue with my Cintiq... In my case, the Cintiq must be the primary Display. I am not sure if Huion is the same. I have a XPen Display that does not have that issue.

NOTE: Poser will remember the location of some screens (by Coordinate Location) which will impact where some windows open, when moved. In those cases, I use the UI Dots to remember Window Locations and then use one for when I use the Tablet and another when I do not. That might solve your issue with the Huion (did not with the Citiq for me)

ratscloset
aka John