This seems like a typical response for beer that still has alcohol in it. The brownish/yellow color in the foam is likely some of the sugars burning off in the heat. How long did you boil for? What's the ABV for that beer? Also, did you allow the beer to cool before adding the lye? (I typically put mine in a closed container in the fridge for 24 hours before I make soap, so I'm adding my lye solution to cold beer.) Those things will make a difference in the lye reaction.
That said, I believe that LaBatt is now owned by AB inBev, who are notorious for using corn syrup in beers. I do think the reaction that you saw is more due to alcohol rather than sugars.