[Update on December 2nd]
We have released a system update that provides iOS13/iPadOS13 support for the BLE-MIDI (Bluetooth LE) connection functionality of Korg controller products. For details on how to download and install the update, please refer to the link given for each product.
Tap the "Settings" app and select "Bluetooth".
If the Bluetooth devices is on the list, tap the "i" button, and the select "Forget This Device".
Then try connecting the Bluetooth MIDI again.