Controller stuck on SOS

When it starts up, it flashes weakly, then gets stuck on flashing SOS. It initially started in the middle of a beatsaber game.

I've tried:

• restarting the headset, turning it off and waiting for a while to see if it would start working again
• removing the battery, waiting for 15 minutes, putting in a new one (which was working on the other controller)
• unpairing and re-pairing, both using the app and the pair settings. If I hold the pair buttons it updates the firmware, and then continues refusing to pair. It has "successfully" updated the firmware ~5 times now, according to the headset. On the app it continues reading as updating firmware.

I haven't tried a fully factory reset, but the other controller is working fine, so I don't suspect it would work. It would be nice to factory reset the CONTROLLER, but the steps that Meta has given out (hold b, press menu button x5) are apparently entirely made up.

Anyone have any other ideas?