Learn all about task permissions
Variety of situations affect who can access a task:
Task in a private project
Task followers and project members
Task in a public project
Task followers, project members, and group members
Those who have access to the parent task
When a task is no shared in any project and only a few followers are added to the task, only those followers would be able to access the task.
Add all the relevant people as followers to the task, so they are notified on important task updates.