I believe, if the controller would continue to supply field weakening current when we are off the throttle, those IPM motors would continue to spin due to the reluctance torque. This could turn out to the same drama as if your throttle would get stuck at half.
"field weakening current", or more accurately, Id (flux-creating current) doesn't directly represent torque.
The torque equation for IPM is this one. Note that if you zero Iq (torque-producing current), Torque goes to zero.
Should the controller lose control, the max current the motor will supply to the battery will be the field weakening current the controller was supplying. Not a problem, except for when the battery fuse blows, as at that point in time the controllers FETs will see the high motor voltage...
Imagine you are coasting at 180kmh in a car, 170% base speed. If the FW collapses now your motor is generating a voltage 170% of the base voltage.
If you are running a 120Vdc battery, at those rpm your motor can generate 204V. The only thing between motor and battery is the fuse, contactor and the body diodes.
So you have a short from 204V to 120V and its energy is fueled by the car inertia. You can't turn off that current because if lows through body diodes. In a super low inertia scenario the current spike will be brief, but in a car, the energy to stop the car from 70% over base speed to base speed has to be dissipated somehow, somewhere.
I heard about shorting phase A,B and C together, that way the energy to slow down the car is dissipated in the motor and MOSFET/IGBT die. That sounds scary, its like pulling the handbrake at 180kmh (and releasing it is not up to you). At least it prevents the silicon to blow with the overvoltage.
A safe(r) aproach should be using a powerstage capable of dealing with your overspeed voltage (to prevent mosfet top and bottom to shoot-through), and a contactor capable of breaking that amount of current. I wonder how this safety issue in handled in automotive applications.
In a bike these could be minor concerns, I dunno. Light FW will at most produce some extra, uncontrolled current flowing to the battery that will last as long as the rider is above base speed, probably not too long. If its extreme the fuse will blow and thats it.