Sorry you are having troubles, but please rest assured it is not a feature of the site.
The only other person to have mentioned this to me is also using ios.
I can not replicate this problem on any windows or android device and the coding of the site is not to log people out each visit.
It would appear it is an OS problem which alas I am not sure I can do anything about.
However I will have a look in various support forums and see if other people are having this issue and see what can be done at either end.
PS: I get that you are annoyed, I would be too.