Hello,
While this isn't my area of expertise, I can offer some general information. Please click on the link Change access levels - Azure DevOps | Microsoft Learn to ensure you're following the correct steps to change the access level.
Please note that Service accounts get added to the default access level. If you set Stakeholder as the default access level, you must add the Azure DevOps service accounts to the Basic or an advanced access level group.
For further queries you may also reach to Get support and provide feedback - Azure DevOps | Microsoft Learn
Regards,
Karlie