The precipitates settling is a possibility, but identifying UV as the cause is hard with all of those variables. Even an above-neutral pH makes EDTA break down fast, and gluconate is lucky to last 24 hours in any tank. I also wonder how much that even effects things given the CEC most of us have in our substrate. That's a question for later though, I guess figuring out if it's happening would be the first important step.
It would be handy to find out what atomic theory has to say about using UV to bust the two iron atoms off of the chelate. Making theory and practice match would be a good first step. Personally, I've got no clue how to figure that out.
I'm guessing testing EDTA would be the most useful. Not the easiest though since it seems to disassociate rapidly at neutral pH. You could either use controls and subtract or do some fancy bits with acids to keep it holding together for longer.
Now testing for it wouldn't be so hard. Shouldn't it just be a matter of dumping a known quantity of iron chelate through various exposure levels of UV sterilization then using EDTA and Eriochrome Black T to pull off a titration test? The less EDTA for a color change, the more is still bonded. Toss in some controls and away you go.