So this is a chinese unit (you failed to mention this - no english docs), I would guess made by
the same kind of manufacturers that do pedal-cycle electric motors and controllers, which means
it probably has fail-safe detection on the throttle potentiometer.
You can test this by disconnecting the ground lead (most negative) from the pot while running
at mid-setting. If the motor revs up to full there is no fail-safe, if it cuts out it has failsafe.
To defeat failsafe its probably enough to replace the pot by a single resistor of the same
value. Then control the wiper voltage directly from the Arduino by low-pass filtered PWM output.
There are other ways to do failsafe which are harder to defeat, but the simple scheme I
describe is able to stop run-away motor speed if a wire falls off or is shorted out.