I’m encountering the same issue. I can connect just fine to Facebook and Twitter. When I try to connect to G+, it does not give me the option of which brand page I want to publicize to. It just selects my main account.
I tried to do the standard recommendations of going to G+ > the brand page > settings > manage apps to disconnect WordPress… but WordPress is not even there. The WP app only shows on my main account manage apps screen and was set to Public.
That’s indeed a known issue with Google’s Authentication flow. We got in touch with them about this, and they’ll update us as soon as they know more about the problem.
In the meantime, you can follow the instructions here to work around the problem:
https://wordpress.org/support/topic/problem-with-publicize-after-updating-to-421/#post-7682854
Google just replied, and the problem is now fixed. You should now be able to link Publicize to your Google+ Page!
Confirmed! Went into settings and the correct G+ page is already activated!
Thanks for the update Jeremy!