Are you using the same token to call the 3 endpoints? If you retrieve the issues successfully, it means the token you are using is a 3-legged token.

• GET account users endpoint: https://developer.api.autodesk.com/hq/v1/accounts/{ACCOUNT_ID}/users only allows 2-Legged token, 3-legged token will give 403: this what I suspect is happening.

• GET project users/user_id maybe be failing if the user assigned to the issue is no longer a member of the project, hence. 403.

• Use the GET project users first to confirm if the assigned user exists int this project before fetching their details