What do you get on the leads coming off the CB feeding the appliance?
A dual-pole CB generally, can only be installed such that the two sets of contacts connect to opposite busses. If they both are connected to the same bus, you'd get what you're seeing. Normally, there's a tab or something to prevent you from installing the CB in a spot where that could happen, forcing it to be on both legs of the main.
Is this new wiring, or something that had been working and stopped?