trader4 says: " It seems like they have established their own neutral,
unconneced to the service neutral, going to the other units. "

My assumption is that this is not a self-establised neutral, but the wiring control (from the indoor evaporator) that tells the outdoor condenser to switch from A/C to heat pump.

Side note: kind of strange to me that the indoor units are 220VAC. Call me paranoid, but I wouldn't have minded a true neutral and only 120VAC to each indoor unit.