A user that has all To-Do related Global Rights is unable to see some todos (even if the user is the owner).
For example, the user is a Team member for a project and is the owner of a To-Do item of a task, but the user does not have rights over the Project (does not have View rights and is not a Participant).
Object/instance-associated rights are mandatory to view the To-Do's even if the user has Global To-Do rights.
Here, the end user is part of the Team but doesn't have View or Participant rights over the Project.
The user needs to have rights over the Project to see project-related To-Dos.