If anyone else wants to know the reason for the damn rope breaking only in the situations I've described, do not ask here. It's all so "obvious" and it's just a matter of "paying attention" that is not even worth asking. For those who usually fish, the lines tend to break every 3-4 hours. It's obvious and always happens. But if you make a bonfire by the river, of course, it will never melt. It's a matter of paying attention. Any fisherman knows this because it's obvious.