After my crash, I had visible damage to the left bottom motor, as the wires got snapped off just as they entered the motor. I have since re-assembled each arm using the side hole with a grommet, and the wires pointed inward.
Ran a motor spin test, and the Number 1 (with Frame 10 setup) twitched and pretty much didn't spin
Next, I switched the ESCs for the motors 1 and 2, and clearly the ESC was not the problem. Had one spare motor after replacing one on the left bottom. I attached it loosely on the ground, and it behaved correctly during the motor spin test.
So although there is no visible damage, the hard landing and sudden stop must have caused some kind of motor problem.
Anyone else experienced this?