I work with stained glass. This is just IMHO but glass, itself, doesn’t sag. A stained glass window will sag in time if it is not reinforced.
The individual pieces of glass do not bend or melt but the entire structure breaks down due to time and the fact that the lead pieces or foiled pieces let go. Oh, poo, this is hard to explain. It’s not the glass that is melting, it is the entire structure that holds the glass, that is sagging because it was not properly reinforced.
If you have seen clear glass windows with this sag then you are seeing really old rolled glass that had this defect to start with. I don’t care where you live but the temperatures just aren’t hot enough to get glass to slump. Unless you are Satan and are living in hell; THOSE fires might be hot enough to slump glass. But in your case; It was pained that way. Does that help?
Gosh, I hope so! I’ve done one window at 2’ x 3’ and reinforced it anyway, due to the inherent slumping of the materials that hold together the stained glass. It’s not the glass, it’s what is holding it together.