You need to mak your new portal as default portal. In admin ui, where you see list of portal, there should a "Make default" link next to your portal.
My portal starts correctly and it displays the before-login page without beeing the default portal. I'm able to login to my portal without making it the default portal. I should be able to logout and to simply return to the page where I was before the login without making this portal the default portal. This is a bug.
The behavior you are asking for is the way it used to work. But the results are not what you imagine. If the page you are viewing is restricted to certain roles, then when you logged out, the next thing you saw was the login page because you no longer had access to the page you were one. And the only way out of the situation was to log back in, go to a page that was not restricted, and log out again. It was a huge hassle. The current behavior is much better.
And desiring a different behavior does not make the current behavior a defect. You can always suggest a different behavior and request it as a new feature.