Additional info: I changed my preferences to open contact sheets when a card is mounted, and that works. Switching it back to open ingest dialog does not work.
I did notice an interesting thing. I can manually open the ingest dialog and see my SD card and ingest normally. However, I don't see the card listed in the navigator. I've attached a screenshot showing my current situation.
Not sure if this is related, but it's an interesting thing I noticed happening since upgrading to Big Sur and Photo Mechanic Version 6.0, build 5404 (e13fb4f).