Certified GKH Gardening Expert
Container bananas will, actually, prefer a good bit of shade throughout the day. It will often get too hot in container in full sun.
Now, the reason for this condition will likely be fungal in nature. Bananas are very susceptible to rot issues.
Since it is in container, the solution can be simple: When you water next time, make sure to replace 1/4 of the water with peroxide. You can do this every few months to keep infections away.
Bananas are heavy feeders, and will require a food with moderate nitrogen, but high phosphorus and potassium. You should apply this once a year in slow release form, or every few weeks if it is liquid.
This article will help you to care for bananas: https://www.gardeningknowhow.com/edible/fruits/banana/growing-cold-hardy-bananas.htm