I would look at the environment on the two opposing surfaces.
It's quite possible that there is enough humidity below the deck and
enough drying of the upper surface to promote cupping & warping.

Second, pressure treated wood has high moisture content. Look at kiln
dried pressure treated.
That's expensive. Let the wood come to a more nearly steady state while
stacked on the porch.
This is time consuming and there will be a high percentage of rejects.

TB