Hi,
Are you able to add these fields to a PDP and edit their values there? I have just tested that myself and I cannot for SP Task List Projects.
What you will find is that even the out of the box fields cannot be edited in a SharePoint Task List project in the normal fashion. These projects do not have the typical check-in / check-out project server functionality, so for that reason the custom fields must be configured specially.
Please check that the fields you are trying to edit do have “Allow editing on Project Details pages for SharePoint Tasks List Projects” enabled and then retest.
Thank you,