OnStar diagnosed it this time as:
P25A2 - eng & trans is not performing as expected
U0101 - ABS issue (brakes)
U0100 - ABS issue (another one)
U0146 - “Ignore Unknown DTC”
Took it into the local dealership AGAIN. This time, they think they've found something... a bad module. I don't know exactly WHICH module but it's p/n is: 84488684. They reset the light and now we wait for the part to arrive for installation.