On Mac, if a window is undocked, and closed, reopening will remember it was undocked and open a separate window again. To re-dock, go to Window menu, and choose "Dock Window 3D/All".
If you've docked it, then closed the tab, and it keeps re-opening as undocked, then it may be some corrupt preferences.
This article goes through how to clean preferences:
https://helpcenter.graphisoft.com/knowledgebase/25464/
James Badcock
Graphisoft Senior Product Manager