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There could be a few reasons why a POST request to MongoDB is creating a document but populating it with null values:

1. Empty fields: If the fields being sent in the POST request are empty or contain null values, the document in MongoDB will also be created with null values.

2. Incorrect data types: If the data types being sent in the POST request do not match the data types expected by MongoDB, the document will be created but the fields may be populated with null values.

3. Incorrect JSON formatting: If the JSON payload being sent in the POST request is not properly formatted, MongoDB may not be able to correctly parse the data and create the document with null values.

4. Authentication issues: If the user making the POST request does not have the necessary permissions to create a document in the specified collection, MongoDB may create the document but without any values.

To resolve the issue, the POST request data should be checked for correctness, and the user making the request should ensure they have the necessary permissions to create a document in the specified collection.