It's a potted plant, have you been leaving it out in the rain the entire time?
I'm curious how deep that brown spot goes. The out part of the pseudostem is a compact assemblage of overlapping arranged leaf sheaths. You can probably pull it back gently and see how deep the brown spot goes.
However, one problem I see with your plant is that the pot is way too small for its size. I can't recommend pot size, since I don't do container gardening, all my banana plants are out in the yard. I'm not sure how bananas do in pots, but if it's possible I'd plant it outside, but probably wait till next year, depending where you live, but for now it looks like it needs immediate transplant into a larger pot.