I’ve never heard of a single key ceasing to function, but there you go. I tried rebooting my laptop and re-pairing the keyboard, and I blew in compressed air for good measure. But sill no tab. (FWIW, I’m running Catalina 10.15.7 on a 2020 Air (again, the tab on the Air is fine); the keyboard is an older battery operated Apple keyboard, simple, and all other keys work brilliantly as they have for years.
Try the keyboard on another device?
If it is also not working on another device, it is probably a defective key. I had this a couple of times on keyboards I used with my tablet on board.
YT was so kind to show me a couple of video recommendations some time ago with videos about how to clean and repair an apple keyboard.
I haven’t watched any of these, and still no idea why they even show up to me, but maybe you could fix it, after watching one of those videos.
Thanks, and sorry for the late reply. Yeah, it was the key, so I ended up buying a new keyboard.