By default, the status manager is the user who originally published the new task to Project Server/Online. If you are using Project templates, the default status manager on all the tasks in the template is the person who created the project. This is how it works.
If you want to change the status manager, follow one of the options 1. Follow the Brian’s instruction using CSOM (https://blogs.msdn.microsoft.com/brismith/2018/05/25/project-online-setting-a-status-manager-with-csom/). 2. Project manager opens the schedule in Project Pro, inserts Status Manager field and changes to his/her name 3. Change the enterprise view of Project Center page include Status Manager field. Click PWA Settings > Looking & Feel > Manage Views > Project … Task Summary view. Insert Status Manager as Displayed fields. Back to Project Center page, change the Status Manager in the customized view