I was having the same issue with my AirPods Pro and iPhone 12 Pro. A lot of connectivity issues but it would work perfectly with my iPhone X.
The following steps have solved this issue for me so far. I hope it does for you too.
Turn off Bluetooth and Hard Reset
The first thing to try is to turn off your Bluetooth and restart your iPhone:
- Turn off Bluetooth by going to Settings > Bluetooth and turning off the Bluetooth toggle.
- Hard reset your iPhone 12. Please note that if you’ve used an iPhone before then the method of force rebooting iPhone 12 mini, iPhone 12, iPhone 12 Pro, and iPhone 12 Pro Max has changed. So pressing the Power/Sleep or Wake button won’t work. Follow these steps to force restart your iPhone 12 mini, iPhone 12, iPhone 12 Pro, and iPhone 12 Pro Max.
- Quickly press and release the Volume Up button.
- Quickly press and release the Volume Down button.
- Press and hold the Side button (Sleep/Wake button) till you see the Apple logo.
Now enable Bluetooth again from Control Center or Settings > Bluetooth.