Welcome to our forum.
I think what I'm seeing is that bromine (which is destroyed by direct sunlight) is reacting the same way with this UV wavelength from the unit. I'm going to experiment with my show tub and switch it to chlorine and see how it goes. I have some pool contracts that have big Delta UV's with no issues, they're all on Chl though. I think an incompatibility with bromine needs to be documented. My manufacturer up the original 9w unit to 18w and there's probably a threshold for the UV breakdown that's in-between there somewhere as I don't recall any customers having issues with the older style unit. Thanks for your input guys.
As for UV, it will use up chlorine because UV breaks down chlorine but the way it breaks apart bather waste is such that it won't necessarily reduce chlorine demand. So a powerful UV system will usually increase, not decrease, chlorine demand (same would be true for bromine demand, but not MPS). The main purpose for a UV system in a residential spa would be to reduce chloramines, but proper dosing of chlorine after a soak usually oxidizes the bather waste and resulting chloramines before the next soak (usually the next day).