"Thomas G. Marshall"
Ok. Hmmmm....my side porch (uncovered, and just a landing for steps
leading into the house, perhaps 5' x 7', is covered with 5/4 PT untreated,
and it was placed tight and never buckled. I'm not saying that this is
the rule, I'm just adding it as empiracle evidence that's made me start to
wonder...

PT is wood. Composites are a mix of wood sawdust and plastics. The movement
will be different so you are comparing different physical properties. With
wood, most of the expansion occurs in one dimension, the board width. With
plastics, it will occur equally in all dimensions.

Other factors are the temperature and humidity swings where you reside and
that of others who may take your empirical evidence and apply it to their
situation that may differ greatly.