sudo nvram bluetoothHostControllerSwitchBehavior = always because I wanted macOS to use only external Bluetooth dongles instead of an internal one. I can no longer find any Bluetooth device when the external dongle is not connected. I already ran
sudo nvram -d bluetoothHostControllerSwitchBehavior
to reset the configuration, restart macOS several times, restart the NVRAM, restart the Bluetooth module, etc.
Is there anything I lost after using these Bluetooth devices with my external dongle? When you now plug in the external key again, find all the devices correctly.
I tried to find the devices with my iPad (Magic Keyboard 2) and recognized it instantly.