It's an expected behaviour once application send the request in POST, pega application will redirect the request and get the details in GET request. You can also check the same at time of accessing pega application directly.
Can you please further describe your use case? What are you attempting to do with the POST to Pega from the calling? Is this something that could be handled by a REST service (such as a case item being created) or another route?
You provided the following via an SR (which was redirected to the support community as design help)
We have a requirement where user can initiate a case in non-pega application and then pass captured information to pega application for further processing. Pega application creates case based on information received, launches harness, user can carry forward work on pega application to complete case flow.
The Pega application follows the standards of HTTP Clients, including web browsers, that when redirected via a 300-level HTTP response code, the request to the endpoint specified in the location header will be a GET instead of a POST.
One of the main benefits is that your client (Pega, the browser, etc) will avoid POST-ing data to the endpoint you did not intend it for. Otherwise, someone could hijack a popular service and do redirects to a malicious service in order to steal data.
You can launch a modal popup or launch a new window which launches a section (either through Flow Action or Open URL in window) and uncheck "Auto-generated HTML" in the section in the HTML tab and you can specify your own HTML in there and launch non pega application from Form action.