maybe you deleted your history? if you do, then the login data will go away as well.
It happens when your browser is set to delete history upon exiting the window. It also happens if you have a "do not track" option selected, or a "Do not keep cookies" option selected, or a "Clear cache on exit" option selected.
The one that it PROBABLY is, is if you have your browser wipe everything when you exit, as it won't save the cache, the cookies, the temporary files, or the history. If you clear your "cache" to speed up browsing, or clear memory on your device, then it kills all information regarding your most recent logins as a safety precaution. :)
You can go into your "Options" or "Internet Options" location in your browser, and then go to "Security" "Privacy" or "Web Content" depending on which browser it is. :)
-Shadow
Thank you so much for the help :)
Omg thank you so much for helping. This helped a lot thank you! ヾ(❀╹◡╹)ﾉ~
Why does my account always log out when I leave mangago. It's so annoying logging in everytime I want to read a manga. Please help me thanks XD