What it comes down to is that it's basically impossible from a process integration perspective. Sure, you can take assays, and you do. But what about the things you aren't looking for in your assays? All you can do is plug it in, cross your fingers, and look at the yield trends. Not only does sulfuric mix in the supply lines, it gets used throughout the entire process. So if you do have some kind of strange contaminant that you didn't test for, maybe it only actually hurts you pre-gate. And the damage ramps up slowly and unevenly. But if your yield tanks, then you have to scramble to figure out what happened, and sulfuric isn't even your first suspect. (Because there are thousands of things like this that can tank your yields.)
What's worse, you're literally over a barrel. Sometimes Plant A has to shut down and can't send you another iso. Do you take an iso from (unqualified) Plant B? Do you crack open a barrel of sulfuric? You have a stock of qualified sulfuric in barrels, for situations like this, but in practice barrels are worse than isos. Metal everywhere.
I'm not saying it's sulfuric in this case. Lots of chemicals have a story like this one, but it would also not surprise me one little bit.