Sorry it wasn’t much help. I don’t know of anything that would do this myself. In fact, I don’t think that a front end ACF form will even work if the method is changed from POST to GET. ACF never looks at $_GET and only looks in $_POST. This change would completely break ACF unless all of the submitted values in $_GET were somehow copied to $_POST on the server when submitted and I don’t know what mechanism may be used to accomplish this change.
I would start by deactivating other plugins and trying a different theme and see if that has an effect on the submit method. After that, I’d be confused. I might look at hosting or anything else running on the server to see what could cause this.
