I literally just used Cmd-Tab to switch back and forth between this desktop Safari window and a full screen Slack window, so it's clearly possible. :) And, I j

I literally just used Cmd-Tab to switch back and forth between this desktop Safari window and a full screen Slack window, so it's clearly possible. :) And, I just confirmed my suspicion: if you want this behavior, go to System Preferences > Mission Control, and check the box for "When switching to an application, switch to a Space with open windows for an application". With that unchecked macOS behaves the way you're describing; with it checked it behaves the way I'm describing.