MyBB Community Forums

Users logging in are logged in after this logon
You are successfully logged in.
Now you will be back to the place you are in.
But after the page is logged, the user is logged in and the user is still recognized as a guest

provide forum url & a test user account so that someone can check

No doubt a cookie issue.

Sounds like a cookie issue to me as well.

https://docs.mybb.com/1.8/development/cookies/