I have a Seneye on my 120 gallon reef. It monitors my pH, temp, and ammonia. Heavy target feeding for me looks like 1/4+ teaspoon of roids, 1/4+ tsp decapsulated brine eggs, 4 cubes oyster eggs, 2 cubes coral food, some times a couple cubes of mini mysis. The Seneye has never registered an ammonia rise. It has shown one in other tanks, so I know it works.
When you say heavy feeding, was it something like that? Or proportionally to what normally goes in the tank would you say it might have been 10x normal? We probably dont' have the same set up so I can't guess whether that would spike your ammonia, but I have never seen a rise off of my Seneye from over feeding my less stocked systems either. I think that in an established tank with as much rock as you have that it would really take a ton of nutrient input to see an ammonia spike and it seems like it would be gone within 12 hours. Check it now and see if it has changed? If yes, then probably that was the cause and you can cross it off the list.
I have at least 30 fish that I feed heavily, countless clean up crew, shrimps and one sea cucumber. I grow Cheato on a reverse daylight cycle under 150 watt led. So this might be why I dont see a rise in ammonia. Since the time that my tank was cycled, every time I have seen a rise in anything other po4, it has been from an external cause. Most of the time traced back to my input water. Which is why I keep going back there.
Sounds like you are sure your input water is clean and you have tight control over your tank. Seems like you have covered everything from new products like a change of brand of GFO, different food, or new hardware that might have leaked so I will pitch in if I get any new ideas. Water changes fix a lot for people. It will at least dilute whatever the issue is and buy you some time to figure out the underlying cause.