Symptoms

 K2 App does not appear in the List Ribbon in SharePoint Online Office 365 subscription.
 

Diagnoses

 1. Confirmed that the user has full control over the site collection.
2. The registration wizard runs with no errors.
3. Creating a new list inherits the behaviour.
4. The Full Toolbar option was selected and NOT the Summary Toolbar option.
 

Resolution 

The client created a new site, trusted the app again and that solved the issue.
Then, they re-added and trusted the app on all problematic sites as well.