I suspect that such canisters are designed to be tolerant of temperature extremes you commonly see in motor vehicles. Still, I would transport them on the floor where the temperatures are lower. Just make sure you know where the catalytic converter is and avoid that location.

And if I was carrying some canisters I would do a sniff test before getting in. If a canister has leaked or vented you should be able to smell it and ventilate. In that case I would avoid using power windows. Probably not much danger of power windows starting a fire but still ...

I have carried a couple one-pound propane canisters in a cross-body toolbox during the Florida summers and never had a problem. At least so far.

It would be interesting to find out what DOT, industry standards, and manufacturer says.