Up Arrow Key: Not working; behaviour of key completely changed.

2019 MacBook Pro 16". I'm not saying I didn't accidentally change a setting using a function key or whatever I have the same problem that was reported by other users on this site that never got a remedy. Suddenly the behaviours of my arrow keys on Apple Full Keyboard AND the MacBook Pro's keyboard have changed. Particularly the UP ARROW key does literally nothing. Launching the virtual on-screen keyboard, it shows the up-arrow key being depressed regardless of which keyboard I use. But, like, right now if as I press the UP ARROW key, it does nothing. Left, Right, and Down keys work as expected. Just UP has changed regardless of me using an Apple Full BT Keyboard, a Keychron BT keyboard, or the native keyboard — so it is NOT the physical keys. I have already tried reseting the SMC and NVRAM [or whatever the acronym]. My gawd, I never realised how much I use the Up Arrow Key until now. Help!

This thread describes the problem exactly.