Publisher won’t save thru a Proxy

We have the Webstories Plugin installed on a site with the following setup:

Admin and WordPress is configured @ https://blog.domain.com
Site address is configured @ https://www.domain.com/blog

The front end of the site is proxied thru an IIS server @ https://www.domain.com/blog but the wp-admin is accessed at https://blog.domain.com/wp-admin

The webstories publisher seems to fail when saving/publishing/deleting any stories as the https://blog.domain.com address as it is making API calls to the proxied (https://www.domain.com/blog/wp-json/web-stories/v1/) address but if we change the Site address to match the WordPress address in the general settings it works fine.

We can launch the publisher and start creating a story. Adding assets to the pages and adding buttons and text just fine. Then upon saving draft the publisher starts sending a series of network requests thru the proxied path with URLs like:

GET https://www.domain.com/blog/wp-json/web-stories/v1/publisher-logos/?_locale=user
POST https://www.domain.com/blog/wp-json/web-stories/v1/status-check/?_locale=user
GET https://www.domain.com/blog/wp-json/web-stories/v1/fonts/?_locale=user

and those all 403 with

{“code”:”rest_cookie_invalid_nonce”,”message”:”Cookie check failed”,”data”:{“status”:403}}

We host WP with WP engine and have an exclusion in place for /wp-json/web-stories/ so it isn’t caching the requests.

Our solution has been to just change the settings, build what we need and then set the settings back as once created, stories themselves are served just fine thru the proxy once built. It’s just the publisher that is causing us the headaches.

Any help would be appreciated.