There are two scenarios where a limited Project Admin can't manage permissions. Only Vault Admins are able to change Project Admin's permissions.
Scenario 1: When a Project Admin role hasn't got all permissions on all content, the Project Admin role is not allowed to set or manage permissions of other roles that are set on Vault permissions.
Project Admins will get the pop-up message below when they try to manage permissions:
Scenario 2: When a Project Admin role hasn't got all permissions on specific content, the Project Admin role is not allowed to set or manage permissions of other roles with custom permissions on that specific content.
Project Admins will get a similar view as below when they manage permissions: