Interestingly enough, hydraulic brakes will also stop working the same way when the temperature of the fluid reaches the boiling point, which reduces the braking pressure significantly. Not a problem in most consumer vehicles in most situations, though. I'm assuming that the regenerative braking is either on or off, and might change quickly. It would be ideal if the mechanical brakes phased in as the regenerative braking phased out - I wonder if that's what they intended to do here, but didn't get it working?