Well i'm not a chemist but there should be some simple tests you can do to at least narrow down the problem.
You say that immediately after dosing you get good iron levels from your tests. Have you tried taking a water sample and letting it sit outside of the tank for 24 hours and then retesting? If it is reacting with something dissolved in the water then the sample should show the same symptoms as the tank.
Also it could be getting adsorbed by the substrate in some manner. You can repeat the above test but this time add in some substrate to the test vial and see if the results change.
A substrate with a high CEC should adsorb nutrients out of the water, that is the whole point of it, so it could be your substrate. Though I don't know how well it absorbs minerals like iron compared to other things.
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