About model and data exchange with 3rd party solutions: Revit, Solibri, dRofus, Bluebeam, structural analysis solutions, and IFC, BCF and DXF/DWG-based exchange, etc.
Now this got really interesting. I used the same files as previously and this time around it worked.
I think the difference was the updated IFC plugin (3434).
It converted all slab parts into objects though.
I have just updated to 3602 and it works too.
There should be a not on the IFC page that the latest version of the IFC plugin is available with the Hotfix 3602.