sounds exactly like what mine did. I updated the BT drivers and everything worked….once. Eventually I discovered that the app would connect just fine with the midi in cable unplugged. (yes, midi, not USB. We already know it priorotizes usb).
And yep, you can lift the ground from a usb cable. Data is carried on pins 2 and 3. Pins 1 and 4/5 are for voltage. Since the usb cable doesn`t power the H9 you could snip the ground (pin 4 if I recall) and be just fine.