No errors but no action either
I can't get this to work despite many hours of trying, there must be a bug but I can't pinpoint it. I'm trying to get a very simple newsletter signup form with just one "email" field on one site to create an Individual and add them to a Group on a CiviCRM installation on another site, but I just can't get them to connect. Frustratingly, I think it connected once but I couldn't reproduce it even after going right back to basics and setting it up again from scratch on a completely different pair of sites. Tearing my hair out here.
I set everything following this tutorial https://civicrm.org/blog/jaapjansma/using-civicrm-form-processor-extension-to-handle-form-submissions-from-an-external
I have confirmed that the form processor works when I feed it from the CiviCRM API explorer V3 (or from the "Try out" link in the processor). On the originating site, I have confirmed that the paths and keys are correct and validated in the "Contact Form 7 CiviCRM integration" plugin with the right permissions. I can see the POST arriving in the Apache log at the CiviCRM end. There are no errors in the CiviCRM log or the Apache logs at either end. I tried installing the CiviCRM "API REST log" extension but it shows nothing.
I tried testing using curl in the suggested format from the bottom of the API explorer V3 window and got the error "Unable to decode supplied JSON". I think that might be a different bug however, because the same query works by other methods.
I tried viewing the contents of the POSTed REST form on https://pipedream.com/ and it looks OK to me, with similar contents to the suggested curl. But there are some subtle differences so I'm attaching pictures of both in case they help.