When an issue is created and an assignee is selected who isn't the creator of the issue, by design, the assignee's role for the issue is set to contributor:
Assignees with a contributor role can't modify the issue status or any of the detail fields.
To allow the assignee to change the issue status and/or details, the assignee's role must be changed from contributor to manager. Once the assignee's role is changed for the issue, the assignee can make changes:
If the issue creator and assignee are the same person, then that person has a manager role which grants them permission to update the Issue. This problem only occurs when a person who isn't the creator is assigned to the Issue. By design, they are given a contributor role.
The assignee's role must be changed from contributor to manager if they need to change the issue status or details.
REF 000001532