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A couple of things:
It is correct that warmed up tire pressure is what matters to driving safely. But for consistency's sake it is cold tire pressure that matters for filling them correctly and then it is presumed that the tires will warm to similar temperatures and correct pressures. All automakers give tire pressures specs for when the tire is cold, not after you have driven it and warmed them up.

Even if it is true that the air volume change is proportional across temperatures it is not true in real life that tire pressure change from different starting temperatures as the tire heats up is proportional.
An under inflated tire will get significantly hotter than a correctly inflated tire because the sidewall flexes more as it rolls to the bottom and flexes as it flattens out. An overloaded tire will also generate more heat.
Both of these conditions will cause a tire to get hotter than it otherwise would and cause the pressure in that tire to change more than a correctly inflated tire will.
But mostly excess heat will cause the tire to come apart, so ultimately if the light is on, check the tires.

And yes I know - we've heard that message somewhere before...

Hope that helps,
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