After Bluetooth pairing with a separate device, it becomes impossible to connect to devices such as iPhones that could previously be connected to as normal.

The maximum number of KORG Bluetooth MIDI devices that can pair with is 4. If this is exceeded, Bluetooth MIDI devices cannot be connected to, even though they are visible.
Please refer to the "I can't connect the Bluetooth devices even though it appears on the list on my iPhone/iPad."