From your description, it definitely sounds as if the wiring, plug or switch on your left hand brake lever is faulty.
If you had swapped the brake connectors over and the opposite lever then activated the regen, it would have been due to a fault inside the front harness multi-connector instead.
I'm now puzzled as to why you say it's your rear brake that's too powerful, as in my experience, most rear brakes are typically connected to the left hand lever, but perhaps yours is different?
As your left lever is faulty (assuming it's the one connected to your rear brake) you should only experience the strong regenerative braking force when the front brake is applied, and not when the rear brake is applied on its own.
Alan