This is a bug I stumbled upon while doing an Open Hand Monk playthrough. It may seem useless at first glance because you could just use a weapon in your right hand, but this doesn't let you use Tavern Brawler and even though Monk gets various ways to use unarmed attacks with a weapon equipped, they either require Ki Points or can only be used once on a target before needing to expend a Ki Point.
0:00 Intro
1:55 How to trigger the bug
2:48 Other quirks/outro
Also yes I'm playing with some mods
Don't judge the fit