Times entered by Project Manager skip approval even with Self Approval De-scoped. You expect the approval task to be assigned to a different user.
The project manager is probably the only team member. Therefore, the system does not find anyone else to be the substitute to approve the project manager's entries, resulting in automatic self approval.
Add someone else that you want to be the approver for the manager's hours to the team and also mark them as the Substitute for Project Responsible. Then the system will find the substitute and assign to them the approval task.
KBA , AP-PRO , Project Processing , How To