Hey! So I had this exact problem, and it was driving me nuts. Turns out, it was because of my browser extensions. I was using an ad blocker, and apparently, it was interfering with Facebook's cookies. I disabled it for Facebook, and the "why is Facebook saying session expired" error stopped.
If you're on desktop, try disabling extensions one by one to see if that helps. On mobile, maybe check if you have any weird apps running in the background.
If you're on desktop, try disabling extensions one by one to see if that helps. On mobile, maybe check if you have any weird apps running in the background.