What creates the maintenance shadow/grayed out section in UMP graphs? What process determines when the shadowed portion appears or disappears?
The maintenance schedule is what creates the shadowed portion of a graph. As long as the schedule exists, the shadow will appear in the graph. When you delete the schedule, UMP no longer tracks the schedule and the time frame will no longer be shaded. If you want to keep the shaded portion in the graph, do not delete the maintenance schedule that creates it.
This functionality is working as designed.