I have a few cases where ifc export does not export valid solids. Attached an example. The solid has no errors in FreeCAD. Export to brep or step and save as FreeCAD file works as well as reloading them. But export to ifc and reload the file does not work. At reload in FreeCAD some faces at the bottom of the solid are missing ! If the ifc file is imported into another application (in my case Allplan) all faces are there but it is no longer a Solid, means Allplan can not build a solid out of the faces in the ifc.
Attached the FreeCAD and the exported IFC