This sounds like the motor protection is disabling the motor, the so called PTC.
When driving a 393 from a VEX brain such as the cortex, the PTC in the motor would last perhaps 5 seconds when stalled. If the motor is receiving 18V, when the motor is stalled that’s going to equate to more current, the PTC will heat up more quickly and the motor may be disabled after only 2 or 3 seconds. Even when the motor is not stalled, higher voltage will cause the PTC to trip sooner.
Lots of useful info on PTC here I posted in a previous life.