When pulling you wheel apart it is easy to drop a washer or screw into the motor and miss it. Once a foreign object gets utterly chewed by the motor, it would be hard to notice any difference between magnet fragments and the foreign body. Im not saying this is the case with you, but it is good to advise people that do repairs of this anyway, so double check tripple check when doing repairs as usually after something gets chewed up in the field may be hard to tell.
The regen you maybe experiencing may not be regen at all. Check your controller phase wires are not damp, or there is any connection between them. Look for insulation wear on all the wires, or bits of magnet that may of got onto the controller. Check all your wires for moisture.
To be honest the GM controllers are crap IMO. Too many people who ride Ebikes with much experience agrees, I hate coming to these forums and reading failures. For very light use they maybe ok.
But I've been a member here on and off with different accounts for years and and put heaps of effort in helping people here. Yet I still see no effort to fix these controller issues. I've seen people come and go, make requests in design and GM has delivered, however when I asked for 100v 12 fet controllers as the default for 48v systems and suggested making a replaceable internal controller part of their battery and not the motor, GM has always failed to deliver. IMO Longer phase wires makes a lot more sense than longer power conductors in order to make regen more forgiving. Electrically, layout wise, one set of conductors, be it, phase or main power cables, is going to have some length to them, they got it upside down IMO. And DC is more efficient transferred over longer cables via PWM and hall timing events than pure DC from a battery.
It all might be cost effective for GM but for $50 or so more for a kit, wouldn't the missing headaches be worth the extra expense?
GM can attract plenty of business with its awesome design looks, it a good selling point, but people are prepared to pay more for reliability.