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Ad Hoc Task Shared with Other Users
When a User creates a new Ad Hoc task without assigning that task to anyone and attaching it to a project, the task is still shared with other Users.
In this case, personal tasks are shared with teams the User is a part of as well as their managers.
 
NOTE:
The only way to remove the extra sharing is to go to the task afterwards and remove the users manually from Task Actions -> Sharing.