Hey bro. No bid deal. It's a very old fan leaf right? 1 of your 1st my guessing. Your plant is 100% healthy, you have no def AT ALL. Its just at the end of a process where it eats last and hasn't got enough energy therefore it's getting eaten by your healthier growth. This is avoidable by doing light defol or not. But make sure you are getting them cola and inside structures light. So maybe open her up a little just around the middle section take them fan leafs off, they've done there job the nodes above them have got it from here.
Good luck bro. Nothing to worry about, she's very healthy