I knew an oddball guy who would have lunch at Quiznos. He said diet coke left out in the heat was very bad for you. Could be exposed to hot sunlight in transit or storage and you drink it later obviously cooled. He would not drink it. it is not very good for you anyway.
Aspartame does change in taste when stored at elevated temps. It isn’t supposed to be any worse for you when it has turned, but it does have an odd, metallic taste.