On a cheap Chinese motor I bought a while back, I had a similar experience. It worked for a brief time, then it completely shorted out.
Since I had nothing to lose, I disassembled the motor.
I found a metal burr, welded across a pair of commutator slots, which judging by the color had come out from the bell housing.
What i think happened is that during stamping the housing was not properly deburred. It was assembled to the rotor, then the motor tested for perhaps a pair of seconds, packed and shipped.
During shipment, the burr must have come lose. When I first applied power to the motor, the loose burr eventually landed on an energized commutator, and the the available energy caused it to weld and short the rotor.