Very much a freak occurance but it might've made a difference if you tugged a bit on the feeder with some tongs. I just recently had a 900g female not fully kill the rat and she started to chew, I tugged on the feeder while lifting the BP gently. She quickly wrapped and constricted.

The vast majority of the time the natural way is just as humane as CO2. If the prey is appropriately sized I have noticed loss of consciousness within 15-20 seconds from all my feedings. Not to mention the feeder isn't feeling too much due to the neuro chemicals that are racing through its body after getting grabbed.