I tried quitting the app while I was editing a Master page, and although it brought up the “Don’t Save, Cancel, Save” dialog, Save didn’t work. I could however cancel, leave the Master page, and then save/quit.
I also had another document open, if that’s a factor. Mac OS 10.13.5.
I’m not able to reproduce this, Ben. Could you give me a few more details about exactly what happens? For me, when I have unsaved changes and the master page editor is open, I hit Command-Q to quit, and the save prompt comes up: if I click Save, the prompt dialog goes away, the master page editor closes (at which point I see an empty layout with no music for a moment), then the window closes; if the other document has unsaved changes, I’m then prompted to save that one, too. When you say that “Save didn’t work”, do you mean that the prompt didn’t go away, or it did go away but the document was closed without saving changes, or what?