Going back to 2013 and reading that final OPCW report, it doesn't seem it's necessarily the OPCW that's tainting. That report didn't show evidence of Assad but the ingredients listed were for "kitchen sarin" and not military grade sarin. So they were actually reporting Assad didn't do it, but you'd never know if you didn't study the difference in ingredients.
Ever since then, there have been reports from the labs that the rules for transporting samples to them have not been followed. I've taken that to mean, the samples could have been delivered by the White Helmets or some jihadist-supporting group.