I contacted Apple, and they gave me this to try (resetting SMC). It applies if you have the T2 chip, which I do, but it didn't work, though it may work for others: I'll follow up with them more later.
"Okay go ahead and do this for me : Notebook computers with the T2 chip
Before resetting the SMC, try these steps:
- Press and hold the power button for 10 seconds, then release the button.
- Wait a few seconds, then press the power button to turn on your Mac.
If the issue persists, follow these steps to reset the SMC:
- On your built-in keyboard, press and hold all of the following keys. Your Mac might turn on.
...Control on the left side of your keyboard
...Option (Alt) on the left side of your keyboard
...Shift on the right side of your keyboard
- Keep holding all three keys for 7 seconds, then press and hold the power button as well. If your Mac is on, it will turn off as you hold the keys.
- Keep holding all four keys for another 7 seconds, then release them.
- Wait a few seconds, then press the power button to turn on your Mac."