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There could be several reasons why using java.net.HTTPClient may result in a 400 Bad Request error while the same URL and body function properly with other tools such as Spring RestTemplate and Postman. Here are a few possible reasons:

1. Inadequate headers: The HTTP request may require specific headers that are not being provided by java.net.HTTPClient. This could be due to differences in the way the client libraries handle headers.

2. Missing authentication token: The API may require an authentication token, which is not being provided by java.net.HTTPClient. This could result in a 400 Bad Request response.

3. Invalid request body format: The request body may have a specific format that is acceptable by Spring RestTemplate and Postman, but not by java.net.HTTPClient. This could be due to differences in the way the client libraries encode the payload.

4. Server-side validation: The API may have validation rules that are not being met by the payload sent from java.net.HTTPClient. This could result in a 400 Bad Request response.

To troubleshoot these issues, one could try checking the API documentation for any specific request requirements, comparing the headers and payload sent by the different client libraries, and analyzing the error response from the server for more information.