Why does DC motor speed controller turn into on/off switch when source voltage is too high?
A likely possibility is that your AC powered power supply just cannot supply the peak current needed to start your DC motor via its controller. It's called stall current and is usually much higher than full load current.

When any power supply cannot supply enough current, its output voltage drops significantly and now you are in the realm of a thousand possibilities. Check that the power supply can deliver full-load AND starting current for the motor.

Also, running it at 21 volts rather than 18 or 19 volts could be enough, in the long run, the harm the controller and/or motor. You cannot assume that because something is seen to run at 19 volts you can put 21 volts on it with impunity.

Hey it works on 21 volts, now lets try 23 volts etc...