The motor/control circuits are isolated from the bike, but there is the battery voltage (30-55 V DC) and an AC component from the motor that could possibly leak out. Most people cannot feel the DC, but add the AC and it will tingle. Wonder if it's going between the two brake levers. That could be bad.

Are the brake levers bare metal or painted/anodized? I'd try to see if my voltmeter could measure anything between each lever on AC/DC. Maybe disconnect the brake sensors.