Let us know how you remedy the situation.
It nice you're able to see behind the siding, if only in one place, there is a problem.
I could believe it the same reason, T1-11 rots prematurely around here. Only nobody knows there is a problem, because they can't see the backside.
In any event, I said from the beginning I'm not a big fan of the z strip for this purpose. I'm sure it could be alright, but rarely are panels stained b/4 they are put up. I have to "LOL" at myself, I tried it to stain the entire panel b/4 putting them up. I found out you never let the sun hit the panel b/4 it goes up, while still wet. I found out the panels take a banana shape, only worse. 8-)