When using the Assign Footprints tool, the preview footprint box is blank for every chosen footprint. These are all footprints from the default KiCad library.
To get this behavior,
I choose the Assign Footprints tool, select any symbol
select any footprint on the right,
right click and select “View selected footprint” or double click the footprint name
I opened an older project and went through the process described above. The footprints DID appear. Exiting the old project and opening the new project that was previously not showing footprints now DOES show them.
I am not sure exactly how this fixed the program, but perhaps this will help someone in the future.
I believe this is a known but intermittent bug where the footprint editor/pcb editor sometimes does not display footprints on Mac. You appear to have found the work-around, which is just restarting KiCad/the editor in question.