If it is browning a bit I'd say it is not getting enough macro ferts. As you know, it can be a rapid grower and does like to suck down nutrients, so I'd consider dosing some nitrates etc.
Just dosing CO2 without the other ferts suggests you are limiting plant growth by restricting those nutrients and my experience with wisteria is that it shows that kind of shortage visibly. Your wisteria took what it found and is now starving for more.
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