you are right !!
Following your instruction it works .
On my Win 10 PC I user PCB-GCODE ver. 220.127.116.11
However , on my other PC I have EAGLE 7.2.0 and PCB_GCODE ver. 18.104.22.168
The problem does not occur with this set-up.
I have, in my configuration, 6 viewer windows open with no problem.
However the windows are reduced to icons on the taskbar.
Thank you for your help,