More people were logged out of their account, but no worries, you can log back in again with the same username + password! If you forgot your password, you can click on the "Forgot password" option at the login screen.
I checked my settings and my browser is not automatically clearing cookies or saved website data when I close it. This is happening on my laptop and on a computer I use at work as well. This happened to me before awhile ago and I think it was a bug that appeared due an upgrade wiki/fandom made. What is the link to report a potential bug to the community staff?
Well in my case I was viewing the Hibike! Euphonium wiki when I was logged out, just the Recent Wiki Activity page. Then I logged back in, but whenever I clicked on an article to edit it (didn't matter which) I got logged out. As soon as I was back to the main page I was logged back in.