Not sure that I can ID what is actually causing it, but I can share my experience of something very similar recently and I only had to cut out the brown parts.
I got some browning on a new shoot about 7 weeks ago, at the beginning of our Summer here in Australia. I initially thought it might have been sun damage to a tender new leaf. When the leaf uncurled, the brown parts were translucent and lifeless. So just in case it was a fungal or bacterial rot, I cut the brown bits off. Thankfully, the leaf survived. The p-stem is now about 30 cm high with no recurrence of the brown bits.
Anyway, I hope that helps and that you won't have to cut the whole p-stem to a stump this time.
https://www.dropbox.com/s/vhya2wpuon...41.54.jpg?dl=0
https://www.dropbox.com/s/ak8qd3vzmx...49.12.jpg?dl=0
https://www.dropbox.com/s/hi7mx4y18m...53.54.jpg?dl=0
This is a pic from about 5 days ago. The leaf I cut is the lowest one on the right hand side. You can't really see the cut edges from here, but it at least shows that the leaf is still happily leafing along.
https://www.dropbox.com/s/qxfk00zv8c...23.45.jpg?dl=0