The following basic procedure is repeated:
Duplicate a previous mockup of a webpage (based on a symbol) and make content changes (i.e. design User Management screen)
Duplicate that mockup, draw a borderless rectangle over the entire page, set to black, 50% transparent, then draw a new rectangle on top of that which forms the foundation of a popup/modal screen (i.e. an edit screen).
Some time later, duplicate that screen in order design a different popup window (i.e. a deletion prompt).
The black 50% rectangle has suddenly changed to white!
If I go back to the previous mockup, I notice that the rectangle appears black as expected but the Background is actually set to white.
This has happened a number of times when duplicating mockups in this project.