Thanks for the input. A section image is below, the hosting level is red. Computation height is standard 3'-0".
The issue persists even with walls with no base offset, brand new rooms, adding room separation lines, only room separation lines, copied rooms, different walls types with or without finish layers...
I have discovered that changing the "room area computation" from "At wall finish" to "At wall center" fixes the issue. Even more evidence that this "donut room" is a bug related problem.
Which is it has to be because I don't believe there is ever a condition in Revit in which the center of a room can be on another side of a bounding element, or the center of the room is the only part not included in the room area.
R2021! everything will be fixed