My husband and I ran into the same thing on our pressure treated deck in Raleigh, and sealing the ends definitely helped slow it down. What finally made a difference for us was a second coat of sealer on the top faces about a month after the first one, once the wood dried out some. Cupping still happens a little in the worst of the summer, but nothing like before, so you are probably fighting the humidity more than anything you did wrong.

Same thing happened to me on a trip to the Wallowas, pinhole right by the valve from who knows what. Now I always blow a little air in the pad at home before I pack it and let it sit overnight, that way if there's a leak I find it in my living room instead of on the ground at 2am. A patch kit lives in my pack now no matter what, it weighs nothing and saves your whole night.