Ok I fixed this - it didn’t really have anything to do with spell power. Your custom spell distribution resulted in an edge case that I wasn’t checking for. With the fight length you have chosen, 5% of your casts as pw:shield, 5% as flash heal, and 90% as greater heal… you would end up with less than 1 full cast of pw:shield and flash heal. Caused some weirdness in the calculations. I added a check for that.
You’ll want to change your spell priority or increase the fight length if you want the optimization to ever consider casting flash heal or pw:shield.