ajax_page_state theme is altered

Problem/Motivation

While working on a feature we discovered that after the first ajax request was made from the Yoast Seo plugin, the drupalSettings had been altered which in itself is normal however the change is that the theme of AjaxPageState is altered to the default frontend theme even though it was supposed to be the default backend theme.

Steps to reproduce

1. Create new content of a content type where the Yoast Seo plugin is used.
2. Verify in the developer console that de theme of the ajax page state is the theme you would expect. drupalSettings.ajaxPageState.theme
3. update a field and see that the Yoast Seo has fired of an Ajax request and got a response.
4. Verify that the theme in the ajax page state has changed to the theme used for determining the SEO score.