“If that piece of butter had been heated to 150°F, it would not have melted” can be read as “that piece of butter has not been heated to 150°F, or it did not melt, or both,” or “it is not the case that both that piece butter has melted and that piece of butter has been heated to 150°F.”
“If that piece of butter had been heated to 150°F, it would not have melted” can be read as “that piece of butter has not been heated to 150°F, or it did not melt, or both,” or “it is not the case that both that piece butter has melted and that piece of butter has been heated to 150°F.”