Your POST became a GET because of an unhandled HTTP redirect.

Your GKE ingress redirected your insecure http:// request to the secure https:// URL. Following this redirect, your requests client automatically changed the method from POST to GET, which is standard, expected web behavior.

You may try to fix the API_URL in your Cloud Run environment variable to use https:// from the start. This prevents the redirect and ensures your POST arrives as intended.

To reliably trace this, inspect the response.history attribute in your Cloud Run client code. This will show the exact redirect that occurred.