Is this really needed? How big is feedback current?
I don't encourage people leave flux on the pcb, but I prefer using "no clean" fluxes that are non-conductive and non-corrosive (but don't trust package labels, not all no-clean fluxes are really no-clean). That's because cleaning flux from underside of ICs can be a challenge.
(of course, for low-leakage applications leaving flux is not an option).