Currently, if a master is open for edit, and you hit the save button, you get this message:
"Modifications made to a master open in a window will not be saved. Close the window and re-save the drawing to record the modifications."
This is super-frustrating to paranoid developers like me who save their work often. With this feature, you have to close the master (and all the group windows and ShapeSheet windows associated with the stuff you are working on), close the master, and completely forget what you were working on.
A better solution would be to ask the user to update the master, then run that code, but allow the windows to remain open, and not break the developer's work-flow.
The current workaround is to copy a shape out of its master container, and edit it on a drawing page. When it is complete, copy it and paste it inside the master container, thereby replacing the old one. Not super hard, but annoying, especially if you are editing lots of masters.
The inability to save open masters causes huge, inconvenient workflow changes, for something that could probably be improved by the Visio team.