12:36 PM
The shared neutral in a multiwire branch circuit can have less current, due to the nature of the circuit. The GFCI sees the imbalance, and interprets it as a ground-fault.
In this situation, if the neutral and grounding bus bars were connected in the last panel. All the current would not be on the neutral, and the first breaker would trip.
The more I think about it, the more I think there's a neutral to ground short in the tub or wiring after the last panel.
That would mean some amount of current would be on the grounding conductor, which would trip the GFCIs. If the OP disconnected the grounding conductor, the path to ground would be lost. When they touch the water, they provide the path to ground. That's why they get a shock.
