Are you seeing the same behavior in all the browsers? Are you getting any errors in the browser's console log or the AEM's error.log when the issue happens?

The IP which you have specified here, is of Author, right?As you said, all of your outgoing traffic goes through PAT, seems like the publisher is not able to communicate back to the author due to this network restriction. Can you please test your FDM submit service on the publisher and check if it w...

Is that unknown IP of the Dispatcher or Author or which other server? please confirm.

The form should be submitted to the Author only, or if you are using FDM, then it should be submitted via FDM on the publisher. You should check where is this unknown IP configured.

Seems like you are using FDM to submit the form? Can you please try publishing the FDM as well from author, and check the issue again?

Can you please share the screenshots of the "AEM DS service settings" and "Apache Sling authentication service" configurations?

Have you configured AEM DS service settings on Publisher?https://experienceleague.adobe.com/docs/experience-manager-64/forms/install-aem-forms/configure-aem-forms/configuring-the-processing-server-url-.html?lang=en 

@nowackem, how are you reader extending the XDPs? We can reader extend the PDFs only, not XDPs. I am using AEM Forms Reader Extensions to reader extend my PDF.

@nowackem Why are you opening the Reader extended PDF in Designer? If you want to make any changes in the form, then do it your XDP, save it again as a Dynamic PDF, and reader extend the same. Thanks!

Please refer to the sample PDF below which has a button to add/remove/open the attachments. It would work in Acrobat Pro, but to make it work in Acrobat Reader DC you need to Reader Extend the PDF. https://drive.google.com/file/d/1KHwifVTKxryfGAcJetjEwbnldCQ1hanm/view?usp=sharing