TMS320F280049: HRMSTEP register cannot be set to any value other than zero for ePWM2 through ePWM8

I am trying to use the auto-scaling function for the high-resolution CMPAHR and CMPBHR registers on ePWM4 and ePWM5.  Nothing I do will allow me to set the HRMSTEP registers in these two ePWM modules.  No matter what I do it always reads back zero, resulting in the high-resolution functionality not working.  If I set the AUTOCONV control to 0 and manually scale by MEP_ScaleFactor, then the high-resolution functionality works.  I have tried using the SFO function to generate MEP_ScaleFactor.  It gives me a valid scale factor and also writes that value to the HRMSTEP register in ePWM1, but does not do anything with the other PWM modules.