You probably have not attached a load. The stepper driver aims for a specific current through the coil and will put out the full voltage as long as this current is not reached, then turn off.
The inductivity of the coil will then act as a low-pass filter on the current. A flyback diode protects the FETs.
It may help to think of the arrangement as a buck converter with the feedback voltage derived from a shunt resistor (i.e. controlling current), combined with an H-bridge (sharing the power FETs).
Note that the stepper driver will expect an inductive load. If you connect a resistive load, you will not get a sensible output either (expect a high frequency square wave).
The load will need a minimum inductivity to give the driver time to switch off after reaching the target current.