If a ticket is assigned to multiple users, only the first user will see the ticket on their assignments page. To recreate, create a new ticket and assign it to User A and then User B. Login as User A and go to the Assignments page. If they set the filter to Mine / All, they will see the ticket. If they create a filter to see all of User B's tickets, they will also see the ticket listed under there. But if you login as User B, they do not show the ticket under Mine / All. They can still find the ticket if they create a filter for User A / All.

I went to do that and noticed that the default Mine > All was set to Responsible Only. I changed it to Assigned or Responsible and that cleared it up. Thanks.