Symptoms

K2 components (K2 Worklist and controls on list/library ribbon) disappear from site collection provisioned with Appit.
 

Diagnoses

This is a known issue where SharePoint Online will stop recognizing Appit and remove elements from the drop-down ribbon on a list/library, as well as removing the K2 Worklist app part.
 

Resolution

Removing and re-adding the Appit app from the affected site collection will cause SharePoint to recognize Appit again, allowing access to K2 artifacts and re-enabling the ability to add the K2 Worklist to the site collection.

Note, do not use the option to uninstall Appit, only remove and re-add the application from the site contents.