Flux "Not recommended for use on printed circuit boards." Why?

Hello everyone,

Going through my spare supplies, I've stumbled across a measure of soldering paste that simply states its not recommended for PCBs. Just for fun, I tested on a throwaway board Ive been depopulating; before I can even remove a component with my heat gun, the flux seems to leave dark stains on the surface that don't clear with standard alcohol. Applying heat to it for any longer completely scorches the surface of PCB down to the first layer of traces.

The labeling states that the solution is superior to rosin core flux for more industrial purposes. How could this flux differ from 'normal' flux and why does it leave such a corrosive trail?

Thankyou,