I'm currently having this problem here. I stay logged in on my other devices but my pc keeps getting logged out and I have no clue why. I have "stay logged in" or "remember me" checked (whichever it is called it is checked).
I have this problem on iOS. It happens more often with Chrome than with Safari, but I get it with both browsers. I have to log back in at least once a week if not more. But, it does this with ALL the sites that I login to, not just Xenforo. Never had an issue on the desktop.