Approx, the iPhone log on/log off issue tracks to a glitch in showing log on/log off on the screen, NOT to the log on/log off function being disabled.
It is also independent of clearing history and cookies, and start and restart attempts to fix.
It is simply a matter of the log on/log off pull down menu hiding the last line or two in its display.
The fix: turn iPhone side ways. It stays undisplayed, until one swipes upward and kind of stretches the displayed menu to reveal the Log on/log off line. Then, turn the iPhone back upright and the Log on/log off line will be displayed. I have no clue why this works. But you can tell the guys at NodeBB that the glitch lies in the code that dictates display of log on/log off, not in the log on/log off routine itself.
Note: The issue does not occur on iMac, or iPad, only on iPhone. And my iPhone display settings are on default.
Know this is low priority, but since a lot of folks use iPhones, the NodeBB guys would likely want to master this at some point, especially since the pull down menu is likely to be added to over time.
Hope this helps.
P.S.: And yes, I can be a bit anal about hunting down issues, as I have worked some with digital systems at times in the past. Old habits die hard. :- )