This is an issue that as far as I can tell is only affecting Google Chrome. And it's sort of random in that it happens sometimes and sometimes not at all. But it's not just my site alone, because I've tested on 6 others site's I help manage (so we're talking difference configurations on all).
I suspect the backend (admincp) is different than the front end (forum) in regards to how logins are processed, but do not know how. All I know is periodically when using Google Chrome, it seems sometimes you'll be repeatedly logged out.
It's very random and that is what has me puzzled. I've ruled out my own home setup, because I've switched PC's and OS's (Windows, Ubuntu). Though notably it does happen more on Ubuntu.