Exact same problem when I upgraded from iPhone X to iPhone 12 with my Pioneer AVH-W4400NEX in my 2015 Honda CRV with Maestro HO2 wiring harness.
Can't clear my old iPhone (or any other devices) from the Bluetooth connections. Tried clearing the bluetooth memory and resetting the whole unit to factory settings. But the bluetooth connections always return when I restart my car.
Reviewed the wiring, tried a Microbypass switch, and the parking/ground "Set On" menu trick. No luck at all.
Any updates from others here??