From this website - https://www.hometips.com/diy-how-to/decking-trex-composite-cutting.html

"It is important to recognize that composite decking expands and
contracts with heat and cold. This fact will greatly influence how you
go about working with the material to reduce waste and make your project
easier.
After purchasing the material, store the composite decking flat and
bundled, if possible, in the area where you will be cutting and
installing it. The decking should be at the same temperature as the area
where it will be installed."working in the morning when the material is still cool will make your work
much easier. As the composite decking heats up it not only expands but
also becomes more pliable. This can make cutting and securing more
difficult. If the decking material begins to get warm to the touch as
the day goes on, you may want to stop work and continue the project the
next morning."