It happens to me all the time. It does seem tied to the clipboard data. I use a lot of plans in my drawings that can take up a lot of memory. I’ve never been able to pinpoint at what point it goes grey however. Restarting Visio seems to fix it every time.
To work around this problem, do not use the Undo command to undo the change that you made to the diagram. Instead, manually perform the action that you require. For example, if you added a shape to your drawing and you want to undo this action, manually delete the shape from your drawing page.