did you leave the last batch of po4 in for too long ?
seems like that might be the case ... last batch got full [of po4] and stopped removing, your system po4 went up, corals adapting, then new GFO introduced and within 2 hours po4 went back to normal [which is no longer normal for your corals]
if that is the case. try feeding some particle foods. there are alot of choices out there. dont overfeed, just a bit daily is enough. your system will be back to normal in 2 weeks.
just guessing what has happened though. think about what u have done differently last 2 months, cant tell exactly what the ISsue is from just tests and results of today.