I have a hallway that connects to a stairwell, with the stairwell area on the 2nd floor and a small area above the 1st floor landing sectioned off as "Open Below" obviously.
When I click on the 1st floor hallway and open the room Structure tab, it shows the absolute elevations as expected (see attached "room1.jpg").
However, if you click on the 1st floor hallway in an area that is "underneath" the Open Below area, the absolute elevations for the same room show up differently, with an effective zero floor depth for the 2nd floor, as seen in "room2.jpg". Also note the relative heights change as well.
It took me awhile to figure this out, because I was trying to correct the Ceiling elevation for what I thought was the hallway area, but it would change the floor above value to match whatever I entered into the Ceiling elevation. I was clicking underneath the Open Below area for convenience, but after fighting for awhile I discovered that where you click when selecting a room to view its properties affects the contents of the structure tab. I don't know if this is a bug or not, but perhaps there should be a message somewhere in the dialog highlighting that the floor structure for the floor above is showing a 0" depth because of the Open Below modifier, but that the room ceiling has varying heights. (Though I know the varying heights message does come up when you actually do have inconsistent floor elevations for the floor above.)