Let permission item managers manage their own permissions.
Add an option on permission items to allow managers to update their own permissions.
If possible, make sure that the last manager can't remove their manage permission to avoid complete lockout from the permission item.
Currently a permission manager can manage other user's permissions but not their own. This block does not give any reasonable security improvement since the manager can just add a group they are part of to give themselves additional permissions. The limitation instead makes it very cumbersome for permission managers to update their own permissions without involving admins or other managers.
A permission item should preferably have an option to enable full self-service by assigned permission managers, including the logged in user. One exception could be that they should not be able to remove their own manage permission if they are the last permission manager.