I'm going to peel off this for awhile, but I've found something new. It may have more to do with what I'm referencing on the duct fitting geometry.
I decided to test placing a linear dimension between the (what I thought were) planar faces that coincide on the duct as well as the fitting, then using RevitLookup to see what a working, manually placed constraint's ReferenceArray churns out.
The planar face on the duct item is the same as what I'm getting via API. But this image shows what it's getting on the fitting.
If I inspect the duct fitting, I cannot find anything in the SymbolGeometry that is a "highlight" visibility, null graphicstyleid, geometryobject type.
I think this means that I'm constraining to a reference plane in the fitting when I do this manually. Maybe I need the view to be set a certain way so that I can get at this information?
I'm gonna go relax by building a drop cheek elbow.