Same issue.
Equipment
Quest 2. Xbox One (non-dongle) controller.
I have gone through this a number of times now:
I'm using using the app on the phone (android) which connects to the headset.
I turn the controller on (slow blink) and set it to pairing mode (fast blink.)
And then fire up the 'Add new controller' option on the phone.
It scans and eventually sees the controller. I select it.
The app now says the controller is paired.
HOWEVER, the controller continues to fast blink through the pairing cycle before dropping into the slow blink cycle and then shutting off. Turning the controller on and attempted to do something in the headset does nothing. The controller never stops blinking which means it doesn't think it's paired to anything. The APP on the phone still thinks the controller is paired to the headset but neither the headset nor the controller do.
I then unpair and try again with similar results. I have tried different locations in the house to account for interference. I have rebooted the headset multiple times (complete shutdown.) And I have swapped the batteries on the controller.
When I observe this process through the lenses of the headset, whenever I tap the controller in the app to ADD it, through the lenses the world environment goes black for about a half a second and then comes back. So the headset is REACTING to the attempt to bluetooth pair with the controller but I never get a chance to confirm the pairing because the environment comes back immediately.