CXone Studio: GetRESTProxy returning 404 on internal API call

Trying to call an internal CXone API from a Studio snippet to fetch custom attributes. I’ve added a GetRESTProxy action pointing to /api/v2/users/{user_id}/attributes. The request seems to fire, but the response object is empty and the status code is 404. I’ve double-checked the user ID variable and it’s populated correctly from the previous step. Here’s the snippet config I’m using:

{
 "action": "GetRESTProxy",
 "parameters": {
 "url": "https://api.mypurecloud.com/api/v2/users/{{user_id}}/attributes",
 "method": "GET",
 "headers": {
 "Authorization": "Bearer {{auth_token}}"
 }
 }
}

I’m using the standard OAuth token passed into the flow. The same call works fine in Postman. Is there something specific about how Studio handles the token or the endpoint path? I’ve tried adding the ?expand=customAttributes query param but it didn’t help. The error message in the Studio trace just says ‘Resource not found’. Not sure if I’m missing a scope or if the URL format is wrong for this context. Any ideas?