Could be a cookie problem geoffw - notwithstanding your avatar's predilection for such.
Problems with being logged in when you shouldn't be, or not being logged in when you should ... are usually cookie related.
Getting "logged off" when clicking New Posts is a frequent occurrence- I get that several times a day. However, if I click on Quick Links and then Today's Posts I'm logged in again- then clicking on New Posts again shows me as logged in.
It behaves as if I have one session time (I'm not sure, perhaps 6 hours) applying to New Posts and another much longer session time- some days- applying to the forum as a whole.
I don't know if this is restricted to iPhone. I do 98% of my forum browsing from my phone.