You're using Merlin firmware, I'm using stock, that may be the difference.
Just out of interest, why

? I took one look at the changelogs and decided on Merlin's and I've even donated (usually the kiss of death to a project

). His support is excellent and he is a very responsive chap.
edit- or are your login details being stored on a session basis by the browser?
Maybe. I do not alter the cookie preferences of any of my installed browsers (after checking natch) and they all behave the same way. I'd expect a 'logout' to deal with session cookies anyway.
Regards
DrT