Effect of switching supply on a Hall effect sensor
I think you are on the right track here. PWM is completely on, completely off pulses - imagine your hall effect sensor powered from this. So no, it needs a constant DC supply. 

What you could do is add a diode and filtering cap to the hall sensor power. This will not produce a completely constant voltage, but may be acceptable. If it still misbehaves, then you are left with adding a linear voltage regulator before it also. This will ensure it gets a constant DC supply, at the expense of dropping some voltage.







simulate this circuit &ndash; Schematic created using CircuitLab