Yep, this problem's been around for awhile....I lost a Ziroli P40 for the same reason. Engine died as a result of a dissolved brown stopper.
I don't know about the "different gasoline" part of the story as this is the first I've heard of it. I heard the problem was related to manufacturing of the stoppers.

The first I heard of this problem was way back in 95/96 time-frame.

I lost my P40 a couple of years after that, believing that the problem had been resolved. Apparently it's not!