I’ve noticed the same thing. If I remember right, the iron acts as a nucleation point for calcium carbonate precipitation. You would also see a drop in Ca and Alk.
I also had carbonate build up on everything in my sump when I was running GFO, also my substrate was clumping.
This is why I don’t like running GFO, it’s real easy to run too much and get your parameters swinging. Best to address why you have a nutrient problem rather than using a bandaid (GFO).