It’s not the temperature. The Pyrex would happily sit in an oven that reached stove flame temps . . . until it turned into a puddle of Pyrex.
It’s that the open flame isn’t heating the Pyrex evenly, the parts that are in contact with the flame are heating up and expanding faster than the parts that aren’t in contact with the flame.
From my 6th grade science class, I was under the impression that Pyrex advantage was that is has a low coefficient of expansion; hence, big changes in temperature do not cause the glass to fracture. Of course, every glassware has it limits.
I’ve never had any problems with any of my Pyrex or GlasBake stuff, but it’s mostly all older than I am. The only newer ones I have are a few smaller dishes and are all Anchor Hocking. I knew they were prone to exploding if they have scratches, which is why my grandmother and I never put them in the dishwasher, and only use plastic or wooden utensils when using them.
But I may not be the best source of “cooking safety”. I still use wooden spoons, wooden cutting boards, the occasional aluminum sauce pan, and an orange juicer made of uranium glass.
I was broiling tomatillos in a pyrex dish. Pulled the dish out and as I turned the liquid that bakes out rolled to what I assume was a hotter part of the dish and it exploded in my hands. And I mean exploded!
The worst part is we have vinyl flooring in our kitchen which now has melted spots all over where the hot glass landed.