autodesk forge – User looses project membership after some time

We are creating users and add them to projects within Autodesk ACC, programmatically via Autodesk Platform Services ACC API. Shortly after creation the user looses the membership in said project.

We are using the POST /users and POST projects/{projectId}/users endpoints of the APS ACC API.

Both request are accepted and confirmed by 201 HTTP-status code. Unfortunately after about 1-5 minutes the created users looses the membership in the project. Normally the membership should be permanent.