when it's ok without controller connected then motor is ok, but with controller connected has resistance then FETs are blown.
Might be just one, might be several. Usually will all be on one phase.
Could be from original problem, or could be from mods.
Unlikely to be shunt mod as I dont' see any real amount of solder on the shunts, and moving htem closer or farther away from each other makes no difference to their resistance.
Withotu knowing what the current reading was after vs before you can't know if there was any change that coudl've caused a failure. If no change in accel or speed probably no change in current either.
Since it happened after some time of riding, best guess is heat, if the FETs were not fully and flatly clamped back to the controller case to shed their heat.