Alternate Host feature does not work

We hosted a webinar today. User A was the main host, and User B was assigned within the webinar set up interface as the alternate host. User B received the email from zoom with the note about being an alternate host. 

When User B tries to click on the Start Webinar button, it says you must sign in as the host to start the webinar. That clearly defeats the purpose of an alternate host being able to start things. And would require you to log in with User A's credentials, which is what we ended up doing. 

User B also tried to just simply join the webinar using the reminder email. That didn't work either. 

Any experience here on how to solve these challenges? Thanks.