Be aware that immersing the rat in the boiling water changes the scent of the rat, so if your python doesn't want to eat a rat that hasn't had the boiling water dip, that's likely the reason. Boiling prey is a standard method for enticing unstarted or non-rodent eating snakes to eat rodents, and I've boiled innumerable thawed rodents; it changes the smell/taste of the rodent considerably.
It's not unhealthy for snakes to eat cooked food?
1.0 Normal Children's Python (2022 - present)
1.0 Normal Ball Python (2019 - 2021)