Form POST method always returns 404 page?

Shopify Partner
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Stumped by this one.
Forms work only with GET, any time you set the method to POST the page (at the same url) returns a 404.

Is there any way to debug why the 404 is even being returned?
I'm guessing it's the fault of some app or something that's part of the store - just need a way to find what and why...