I just had an issue where I “lost” Stylus RMX window after choosing “Allow window to be resized” so that the instrument window just opened outside the visible screen area. After reinstalling Stylus RMS (and Cubase 11 Pro) I figured out that the problem is that the window is not visible. While trying to use Cubase 11 Window manager (Window -menu) I couldn’t reset the layout so that the window could get visible again.
Solution was that when the Stylus RMX window was activated, clicking windows “option” -key (secondary menu) from keyboard opened the VST host window and allowed me to disable the “Allow window to be resized” and the window got visible again.
I think the “Allow window to be resized” option is great (not perfect, there are still rendering issues with most of the plugins) but they should be allowed to be reset via the Window -manager of Cubase, just not to get lost between projects.