Anytime I navigate away from MNet (I know, I do try not to do it, but sometimes I need to look at other sites 'kay? 
) I'm logged out and have to log back in again. Even if I have two tabs running, it happens when I switch from one to the other.
I've tried it on a variety of devices and browsers;
- iPhone running latest software update (7.1?) and using Safari & Chrome.
- Kindle Fire using Chrome and Silk
- Laptop running Windows 7 and using Chrome & Firefox. I couldn't bring myself to try IE.
It's not the end of the world, but it is annoying.
Tech did sort it last time, but whatever he/she/they have upgraded has reset it for me (at least!).