missing / erroring sitemaps

In our webshop we have, we make use of rank math; it generates 2 sitemaps for us on the sitemap_index.xml which I’ve listed in the sensitive data. In google search console, sitemap [1] is read properly whilst [2] has an error, which I’ve personally confirmed is an incorrectly set http header:

Incorrect http header content-type: “text/html; charset=UTF-8” (expected: “application/xml”)
Is the error it gives back from https://www.xml-sitemaps.com/validate-xml-sitemap.html

I don’t see any way of changing this header by hand, as it is presumably auto-generated by rank math?

Aside from this issue, there is a big bunch of sitemaps missing, most of the sitemap settings in rank math are turned on like posts, landing pages and categories; all of which are not showing in the sitemap_index.xml of the website.