CXone Data Action JSON mapping fails on nested response objects

I am building a Data Action in CXone Architect to fetch user profile data from an external REST API. The call itself works fine, returning a 200 OK. The issue arises when trying to map the JSON response to local variables. The API returns a nested structure, and the standard mapping syntax seems to ignore the inner object.

Here is the response payload from the API:

{
 "status": "success",
 "data": {
 "user_id": "12345",
 "preferences": {
 "language": "en-US",
 "timezone": "Europe/Berlin"
 }
 }
}

I have tried setting the mapping field to data.preferences.language and data\preferences\language. Both result in the variable being null after the action completes. The documentation is vague about dot notation support for deep nesting. Is there a specific syntax required for nested objects in Data Actions, or do I need to flatten the JSON before mapping? I have verified the API key and endpoint in the request configuration. The request headers are set correctly. No errors are logged in the execution trace, just empty variables.