I've observed some additional behaviour that might hone things in a bit more. Let's say I have 10 different users.
If I login with user 1 after clearing the browser cookies and cache, it logs in fine. Then I logout.
If I login then with user 2, it displays user 1's profile information. I logout and log back in, and it displays user 2's information correctly.
If I login as user 3, repeat all the quirkiness as in the previous case.
If I login as user 1 OR 2 at this point, however, the profile displays correctly after only one login.
So it seems that the quirkiness occurs after logging in more than one user, and after logging in any ADDITIONAL new users to the session. But once they've been logged in at least once properly, then you can switch between them without any problems.
Do I basically need someone to take a look at my site configuration? I'm really at a loss here for what to do...
I was able to solve this problem by changing two things:
1. I had a web link in a site-wide navigation menu to the index page of the secure micro-site, which was using the $modx->user->isAuthenticated('web') user authentication approach. The url in the web link was http://www.sitename.com/micro-site/index.html. This, I believe, was screwing up the context authentication or session state, somehow. I changed it to the site relative path: [[~###]], and it seemed to worked better.
2. I switched from using authentication with $modx->user->isAuthenticated('web') to resource groups and ACLs. Things are working properly now. No more double logins. Thank god.
