The regen should kick in separate from that switch - taking your foot off the accelerator should kick it on and it should stay on even if you hit the actual brake pedal. So stuck in either position, they should be seeing that.
Best I can tell, the car manages regen based on drivetrain load. The car estimates, from speed, the amount of energy in play versus what's been "requested" by the accelerator and when there is an excess in that calculation, it kicks into regen mode.
Looking in the maintenance guide for this, there is a regenerative braking calibration mode in the scan tool, regen itself is managed by the TCM, so you may want to check all the relays and fuses that associate with it. It coordinates with ABS as well.
The guide also suggests it is possible for the system to lose some of the stored parameters and throw a fault code as a result. It notes that certain brake system faults can cause it to simply fail over to friction braking only - many of the components that help manage the transition from friction to regen are being monitored and recurring faults will shut it it off. That should trigger a code, not sure if it would actually trigger an indicator on the cluster if "braking" still works.
"If the ABS module detects that the brake booster is not able to build adequate hydraulic brake pressure in the master cylinder, the brake system transitions from regenerative braking mode into friction braking mode. While in friction braking mode, the ABS module continues to test the brake booster. If the brake booster begins to generate the expected amount of pressure in the hydraulic system, the brake system recovers into regenerative braking mode. If the brake system has to transition from regenerative braking mode to friction braking mode 3 times in the same ignition cycle because of a brake booster performance fault, the brake system downgrades to friction braking for the rest of the ignition cycle."
I also learned something nice:
Because much of the brake torque required to stop the vehicle can be achieved through regenerative braking, it is not always necessary for the conventional friction brakes to be applied. The brake pedal uses a pedal feel simulator that acts against a curved bracket to give the driver a simulated pedal resistance as the brake pedal is applied. The ABS module uses the driver input on the brake pedal, wheel speed sensor input and the lateral deceleration messages from the RCM to determine the amount of deceleration the driver is requesting and which brake mode is necessary.