From what I've found with my Biab brewing is you definitely won't melt that bag maybe some minor scorch marks but no melting.I have some, but haven’t used them yet because I know they are no match for the skin shredding torpedo screen in my kettle, despite being nylon. I’m going to try a mash soon when I get a kettle with a false bottom so I don’t melt the bottom of the bag. All things have a priority, and right now that’s the kitchen remodeling.
Think of it like throwing a plastic bottle filled with water on the fire
The water in the bottle prevents the plastic from melting.