Been using RO for a little over 2 years, and since ditching the tap water, I've only ever used Calcium Chloride, Gypsum, and baking soda (for dark beers). But I've always dumped everything from the boil kettle (tho hops are bagged), so maybe that's why not using nutrients for RO beer wort hasn't caused me any noticeable problems?
I do basically the same. Everything in the BK goes into the FV. I do add the hops directly to the kettle, unless it's more than a few ounces. I rack through a stainless colander to catch some of the hop matter, which helps aerate the wort.
I might look into the ionic zinc and just ditch the nutrients altogether. I can't be sure if over-nutrition is adding off-flavors, but I want to eliminate as many potential factors as I can. I don't have the time or patience to remove one factor, brew, check, remove the next factor, brew, check, ad nauseum. I need to step up my game, change some things. So if it's not needed, out it goes.
Did you taste-test the beer before or at the time you bottled it? Did it have any off flavors then?
That may help pointing to whether it's the fermentation or packaging (bottling) that causes the issue.
Yes, the hydro sample tasted fine just before bottling. Thus, the off-flavor could be developing in the bottles, or it was latent and not noticeable at the time. In any case, bottles are going to get some serious scrubbing and sanitizing before next batch.