Areas need to be visible on an Area Plan within plan regions when the area is above or below the associated level.
The general intent of Areas in Revit, as I and others have investigated, is to be created on an area plan at that associated level, separate from groups and model elements. But, as is usually the case, we need to use areas in a different way.
In our multi-family workflow, areas are included in our unit groups to simplify the workflow and avoid a lot of rework, which works great for levels with no steps in grading. However, when a group is moved up our down even a fraction of an inch, we need the area for that group to show in the area plan, and typically the tool to show graphics properly is a Plan Region. Unfortunately, unlike rooms and spaces, areas are 2-dimensional and will only show at the level of the associated view. It is possible the View Range may need to be considered in affecting the visibility too.
In the attached image, you'll see two units and areas, one hosted on 1st floor, another hosted on a level 2' below. The view range for the plan regions have been set properly for each view yet the areas do not show.