Had a look on GitLab and was confused as I couldn’t find a KiCad project.
The daughter board 3D model was designed using Fusion 360.
I followed your advice and loaded the step file into FreeCAD. Interestingly it kept the colours from the models I’d uploaded to Fusion 360 but not the bodies created in Fusion itself.
I exported from FreeCAD to another step file and then changed the footprint 3D model in KiCad to this new version. In KiCad it looked as in FreeCAD. This then exported from KiCad without errors and uploaded into Fusion ok.
So I’ve something that I can work with but there is still something not quite right with the Fusion 360 export/KiCad import… or at least the KiCad PCB 3D export. I seem to need an idoit’s guide on how to report the bug
Interesting. I initially tried that and ended up at a page asking me to sign up with GitLab. When I did so I then ended up with lots of different projects but couldn’t fing KiCad.
Following your suggestion I then fired up KiCad and tried again. This time it opened up correctly with the page to enter a bug report. So I was only slightly stupid…