I looked into the x-request-id you provided, and I do see that your request is hitting our server as a GET, which you noted and would explain why you are receiving a 404.
That said, there shouldn’t be any way for Shopify to redefine requests from a client - if you send a POST, it’ll be received as a POST, if you send a GET it’ll be received as a GET. Can you expand a bit on what you’ve found that is indicating that we are changing your HTTP method at the server side? I didn’t see anything to match that in our logs but am open to investigate further.