I have a similar problem with solids created in Sketchup. In Sketchup, 3D cubatures (solids) are generated on the basis of polygons (from AGP) and these are returned to the GDB in ArcGIS PRo via the export interface via 3D-objects (e.g. DAE files). There they are displayed correctly as multipatches. But a volume calculation, for example, fails because the geometries are said to be not closed. I don't know why that could be. A subsequent workaround with the "Enclose Multipatch" tool creates a new feature class with closed multipatches. However, no attribute fields are transferred, which have to be added afterwards in a further process. That's annoying.