AEM form submission to Marketo

I hope the collective experience on the community can help identify the root cause for something we are running in to.

I have a customer who, for reasons that are not relevant here, has two production Marketo instances and for some of the webpages on their website forms need to be submitted to both instances. The chosen solution is to have the Marketo form from the first instance visible on the page and on success do an API submission to the second instance using the submit form endpoint: https://developers.marketo.com/rest-api/lead-database/leads/#submit_form

From the test AEM environment (without any webserver and / or cloud machinery in front of it) both submissions work as expected, but on the live domain the API request seemingly is not reaching AEM from the dispatcher.

Has anyone seen this issue before?