Hi Mark,
I had this happen many times when I receive 'naners shipped to me. Due to the fluids from the pstem and the heat, it kind of 'cooks' up the tip of the pstem. As long as it is not fungal or bacterial, they can recover once it stabilizes. You can cut off the rot but sometimes it may continue until it reaches the corm. With strong plants, the corm is very resilient and will grow again. You shouldn't worry too much. All should be fine in due time.
Cheers
