This does happen to me sometimes. I'm a consultant so I end up having to use customer gmail accounts and my personal account. So of course it always logs in with my personal since the browser logs in with that. Then I add this customer. Sometimes when I click to switch accounts it keeps sending me back to my personal account. I'm sure someone from the GMAIL team is on HN so they might see this.
That list is totally broken, it won't update when you get logged out of one of your accounts, and like you've seen if you try to go to one of those accounts, you get bumped over to the one you're still connected to. If you do the "add account" thing and add the one that's not working, you'll be authenticated and able to switch back and forth for a while.
On Firefox I click the user icon, choose my work account and my personal account loads.
On Chrome I click the user icon, choose my personal account and my work account loads.
It's so fucking frustrating. I've cleared cookies, cache and history completely and the problem just comes back later that day.