I had one of those......freaked me out
It is called a choke .
The new leaf or flower cannot travel up the stem.......it just keeps pushing till it pops out the side, quite common with some plants
it's usually the flower though.
Remedy.....cut the stem at the choke to unleash it.
The plant will sort itself out in a week or so.
Or you can do light surgery and unravel it
You can also do a choke search in the banana wiki.
Photo of choke unraveled and the bulge in the stem.
These are pics of my last choke.
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Note the fly on it.
Just slice the stem.
