I often have multiple sketches open in IDE 1.8.13 Mac OS 11.2.1. However, there is no 'window' menu to select a sketch, you have to minimize or move the other sketches to find the one you want. Usually the window menu appears to the left of the Help menu. Can a window menu be added?
I think that the problem might be that it's actually one instance of the application that you are running.
Do you have the same problem if you double-click a sketch in file explorer (or whatever you have to do on the MAC), next double-click another one etc.
PS Not a MAC user.