No, PH flux is just another name for "I dont know what the fuck is going on" u will see lots of people resort to telling u its PH fluctuations, but never believe that. An unstable PH will only prevent the plant from certain nutes, which in turn will cause nute def's, but not unexplained rust spots.
The spots are actually a "leaf spot fungus", 99% positive. If u keep ur temps and humidity in check it will only grow very slowly, but there is no cure once its enters the plant.
U should treat with fungicides if u have other plants, but if u dont just grow it out. Afterwards disinfect all tools, floors. walls, pots, etc... or it WILL COME BACK!!!.
P.S. plant attacking fungi wont hurt humans at all. Trust me, if u eat fruit and vegetables, u eat plant attacking fungi.
Edit, it wont hurt the buds at all, it may cut yields a little, but it would have to get out of control to do serious damage. Indoors u can control humidity and temps so it shouldnt get that bad.
If u want conformation from a different source, just google "septoria leaf spot". U wont find many pics of cannabis with fungi (probably due to people not knowing what it is when they get it) but there is a whole gallery of other plants with the same shit happening.