I use the Unisub placemats and in my experience they do warp slightly. Although I have a solution that seems to be working well for me. I press them at 180c for 120s white face up, then press them white face down for 60s before letting them to cool on a flat surface white side up. Once cool, they're good to go an warp free. May sound a hassle / time consuming, but I am small volume so fine for time.
Hope this helps
