PWM Motor Driver output voltage is enormously load - dependent
With motor switching, you should think in terms of current instead of voltage.  Consider what happens when you are running an inductive load (your motor) under load.  When you turn on the FETs, the output terminal is pulled to ground and current flows through your motor.  When you turn the FETs off, the motor continues to conduct current in the same direction, so the current flows through the diode, turning it on so the voltage across the motor quickly changes from 24 volts to one diode drop, then to zero volts if the switch is left off long enough for the motor to stop conducting.
Now consider when you are running with no load.  The FETs turn on and pull the output hard to ground.  But when they turn off, but there is nothing to pull their drains back up to 24 volts except the diode leakage and whatever else you have in parallel to the motor.  If you were to look at voltage waveforms with a light load, you would see not a square wave but a ramp on the &quot;off&quot; half-cycle.  If you were to add pull-up resistors, you would then see the average that you were looking for.