Long shot but you might’ve solved the nginx x-forwarded header multiple times without seeing it if the nginx proxy is behind the same bridged docker desktop system setup on your minipc then the visitors IP will show as 192.168.65.1 to the nginx proxy too so you forward that same ip to jellyfin even when it works.
An easy check would be to spin up nginx as a vm instead of a nginx reverse proxy docker image and see if it works.
No real solution, but a lot of chatter regarding that:
github.com/NginxProxyManager/…/674