Currently, switching to a saved tab group opens a new window. I can see how this might be useful for users who want to "call up" a saved session, then "shut it down," all while having their main window alive. I am currently trying to see if I can adjust to this workflow.
But, I much prefer Safari's implementation where switching to a tab group loads the group in the existing window. I usually keep a single browser window open one a specific desktop in Mission Control, so my muscle memory is to switch to that same window whenever I want to use the browser. Perhaps a toggle between these two modes, e.g. "Open tab group in new window"?