Happens on both my devices, it used to store them but neither 'Android Browser' nor 'Opera Mobile' store them for very long any more
Given there is not a lot else on the phone than that and Strava who knows.
The Nexus plays better but still looses the logon, though it is easier on that.
Still nice large permanent boxes and button at the top on the not log in skin would help. I know when I used VBB3 bitd we used to have that feature (not for mobile or made large, just to make it easy and obvious)
This site has it at the bottom on both normal and mobile templatehttp://forum.myblogsite.nl/phpBB3/
Even better would be to integrate (if possible with phpBB I don't know) the common login-with option of Google, Facebook, Twitter. For Android users Google-login would be a one push login normally.
hmmm off to look.....
This sort of thing http://docs.oneall.com/plugins/guide/so ... gin-phpbb/
no looked at price just found the site