This approach used to work 100% of the time but recently the behaviour changed. Recently tapping 'back' often but not always results in the forum navigation links not showing (as if one is not logged in). One or more 'back' is required or leaving the forum completely and next going there from scratch.
Not quite the same but if I have a tab open and havent used it for a few hours ( <10) occasionally I will be logged out on that tab even if I am logged in on a different tab.
I'm not aware of any changes that could have changed that behaviour (and it's a really strange one), are you sure is not related to the browser you are using? Sometimes mobile browsers have a really aggressive cache.