I realize you are talking about whether or not GFO leaches back into the water after it is exhausted. But another aspect of the effect of GFO being exhausted that I worry could be harmful, is that there seems to be a relatively rapid rise and fall of the available po4 in the water column when GFO is used.
I am thinking of this scenario where let's say there is .1 ppm of po4 available in the water column before the GFO is employed. Then while the GFO strips out the po4 the total available po4 in the water column goes down to maybe .05 or lower, but when the GFO is exhausted the po4 in the water column goes back up to .1 or so ppm.
I think this rapid rise and fall can happen pretty quickly if there is enough GFO used, with enough flow through the GFO canister and if there is a constant source of po4 that is contributing to the po4 level in the water column, like detritus compacted into the sandbed or po4 leaching out of the argonite or constant over feeding.
I think the obvious misuse of GFO use is stripping the water of all po4 and we think of that as the thing that can cause an issue with GFO use, but I think that rapid rise and fall cycle of GFO use can also cause harm to coral.