For the English Leather in the plastic bottle, I had one of those and it went off on me and I think it was due to the plastic. I say that because I have vintage EL in glass that seems fine after many years.
I agree that plastic could be to blame for some of the vintage AS smelling off. Most Avon came in glass bottles. But a few of their bottles were plastic. I have a couple in plastic and they smell different from the same scent that was stored in glass for the same # of years. It doesn't smell foul or spoiled, it just doesn't smell like it is supposed to. Plastic is not a good container for long term storage, It leaches chemicals into the AS over time. That's why most all of my AS has been transferred to glass bottles.