I think I've figured out the issue & I think currently, there is no solution that Autodesk has implemented. I do not know if Autodesk is even aware of this but I'm hoping so.
[edited] Note, this is using v18, I do not know if this issue is corrected in v19 or v20 at this time.
Description: When a beam is on a level (say Roof Level) & perfectly horizontal, you can select the beam with a spot elevation tag & get the elevation point of the beam. The tag can then be set to "Bottom Elevation" to get the elevation of the bottom of the beam in question.
Issue #1: When a beam is on a roof slope, you can select the beam with a spot elevation at each end of the beam to get the elevation of the TOP of that beam at each end. When you set the tag to "Bottom Elevation", the tag goes blank.
My belief is because the calculation in the program is designed to select the point of the beam directly below the point selected & not perpendicular with the point selected. Thus the tag can't read the bottom elevation if the beam isn't there (the bottom end of the slope) & the tag can't read the bottom elevation of the beam at the top end of the slope because it isn't at the end of the beam.
Issue #2: When a joist is on the beam (Level or sloped), you can select the joist at the point of bearing on the beam however, when selecting the "Bottom Elevation" the tag goes blank.
My belief is because the calculation in the program does not differentiate between the joist's bottom edge & the beam's top edge or bearing point of joist & therefore can't calculate the "Bottom Elevation" because it isn't at the end of the beam bearing the joists to show "Top Elevation" of the beam or "Bottom Elevation" of the joist.
... I think that makes sense ...