Using linux and a Gimp 3.0 appimage. Going into multi-window mode
I do not get the "linked" effect, the toolbox/tooloptions dock does not move with the image window but both dock "hide" if a second image is open. They come back with the first image active. Very strange. I did look into preferences -> window management and nothing there that seems to fix that problem.
Since you are using a Mac version I recommend posting the details as a bug https://gitlab.gnome.org/GNOME/gimp/-/issues
I do not get the "linked" effect, the toolbox/tooloptions dock does not move with the image window but both dock "hide" if a second image is open. They come back with the first image active. Very strange. I did look into preferences -> window management and nothing there that seems to fix that problem.
Since you are using a Mac version I recommend posting the details as a bug https://gitlab.gnome.org/GNOME/gimp/-/issues