I ripped another Glossy set. I didn't find a brown back but about one third into the set I found these three cards back-to-back.
It's probably tough to tell from the scan but the Bill Doran has brown on its right edge and a little on its top edge. The front is very off-center in the opposite directions so whatever cards were to the right and above it on the sheet were clearly brown backs. Here's a better view:
So this really confuses matters. I'm not 100% ready to rule out some sort of environmental damage but the evidence is certainly pointing otherwise. The end card in this particular set, with its back against the tin, was a white back. The brown on the Doran matches up perfectly with the amount of off-centering on the front of the card. Plus, it was found well inside the set and surrounded on both sides by white backs.
Yet, why would some portions of a sheet be brown backs and other portions be white backs? That makes almost no sense. Surely there should be uniformity among sheets, right?
I can't make heads or tails of this.
Arthur