Currently, once one closes Start workbench document tab, activating Start workbench won't bring it back anymore. Likely that is not intended behavior?
This is exactly the intended behaviour. Many people who use Start as their default wb don't want to see again the StartPage after they closed it and double-clicked e.g. a PD feature for editing because after editing FreeCAD by default switches back to wb that was active before.
People who want to see again the StartPage must click (not very obvious) on the blue right-arrow button.