I have a Wyeast smack pack of Lambic blend in my fridge that I bought a month ago before I understood the complexity of brewing a Lambic. I had it next to some beers that I have really enjoyed previously (L'infusée by Brasserie du Monde, and an Imperial pale ale homebrew) and when I opened them both were not as good as they had been. I'm wondering whether having the Lambic pack in with them for a long period could have affected the flavours.
Beers do not easily get infected from the outside. The only times I can confirm infection from the outside is corked beers where the cork shrunk due to old age.
Imperial Pale Ale sounds like a hoppy beer. Chances are the hops just fell out due to age. I think the flavours of the other beer also dropped out due to age.
Hoppier beers should be enjoyed within a short time after brewing. The hop will not age well after conditioning and lose flavor/body over time. More than likely, you are simply tasting old beer. Im not really familiar with the other Canadian beer, but I know it to be a herb/spiced beer, so one could conclude that the herbs/spices would change over time. I know my pumpkin and holiday beers, which are spiced, change after a few months if there is left over from the holidays.
As stated above, no idea how a sealed yeast could effect an unopened beer bottle. Mad science happening if so.