Hi!  I am trying to map the 3D features correctly here according to
 https://wiki.openstreetmap.org/wiki/Simple_3D_Buildings

There is a building:part interior to the building: https://www.openstreetmap.org/way/317206159

When you've got one part, according to the doc: "The entire building outline should be filled with building:part=* areas, tagged with their respective height and other attributes."

Previously, the existing polygon was tagged building:part=yes and there was no footprint at all, so I stripped the building:part=yes from the footprint and added another way that is the lower elevation building:part=yes guy.  

Does that make sense?  According to the doc, you need a footprint, and separately if you add parts you need the union of the parts to cover the footprint, otherwise 3D renderers get mixed up on when to show parts vs footprints. 