I picked a material that when annealed has a higher heat deflection temperature but there are still limits when talking about a loaded mag with spring pressure pushing rounds up (and feed lips outwardly). How hot, how much force and duration all play in so hard to answer exactly.
Without cranking up the price point further to chase a more costly materials I felt this material was a reasonable set of properties for normal use and you won't loose a mag leaving it in a closed vehicle in a gun case or such. If there is a loaded mag in the gun the gun will support the mag from letting the feed lips flex outwardly. The most demanding situation is a loose loaded mag left to bake in the sun along with the hot ambient temps of the vehicle. I'm not particularly worried about a mag in a gun bag or gun case.
For example I purposely put a loaded 223 mag on my dashboard (hot truck, black dash, black mag in sun) for a month+ this spring and eventually the feed lips did start to distort under the spring pressure pushing up the rounds. To experiment I subsequently put a spacer in the mag (the right width) and put a clamp on the mag and left it on my dash again and it warped back into place. I don't recommend conducting the same experiment

but I was curious.
So don't store mags purposely on your dash for extended periods or such but otherwise I think you'll be fine in normal use. If you have an issue reach out.