Tab key on Magic Keyboard will not move between cells in Numbers

Make sure that in your Accessibility settings you have Full Keyboard Access turned OFF.

Thanks for posting this. My Magic Trackpad got broken when a tornado demolished our house, though my MacMini and Magic Keyboard survived. I tried connecting an older touch pad and couldn't get it to work, but I figured out how to enable the Full Keyboard so that I could navigate and login the macOS and eventually I got the old touch pad to work. But I had completely forgotten about having enabled the Full Keyboard under accessibility and didn't realize that this was affecting the keyboard behavior in certain apps. I was considering buying another Magic Keyboard, but thought I'd do some research first and came across your answer. Thank you so much for posting this. You saved me from spending $199 for a new Magic Keyboard... 🤓