Known issue (and already fixed for the next version). See Footprint Editor: 3D Models of PCB footprint borders crop down to a tiny shrunken size (#14246) · Issues · KiCad / KiCad Source Code / kicad · GitLab and Footprint Editor: 3D Models of PCB footprint shrunk (became tiny)
Workaround: Make sure F.Cu layer is visible in PCB editor.