It's important to note first that while OnSong provides a screen for connecting and disconnecting Bluetooth MIDI devices, that screen is provided by the operating system and the connection between the devices is not managed by OnSong. If you have updated iOS/iPadOS and your Yamaha MD-BT01 Bluetooth device stops working, it's likely an incompatibility with the hardware.
You'll need to update the firmware of the Bluetooth MIDI adapter so that it is compatible with changes in iOS/iPadOS 14. You can do this using the Yamaha-provided app and following the directions on this Yamaha support site.
If you continue to experience issues, please contact Yamaha and/or Apple for assistance. Again, OnSong uses any connected CoreMIDI device so contacting the manufacturer of these devices is paramount to resolving the issue.
As always, we recommend not updated to the latest iOS/iPadOS version until you can confirm that all software app developers and hardware manufacturers have confirmed interoperability with the update. Unless you need specific features in operating system updates, it's best to wait if your live performance depends on your iPad or iPhone device.