The sitemap uses the WordPress core URL settings located under Admin > Settings > General
Does the WordPress and/or site URLs use http?
Changing these URLs may cause unexpected behavior. For the health and safety of your site, we recommend making a complete backup of your website before changing these URLs. Learn more about the benefits of regular backups.
If you clear your cache (server, plugin/theme, or CDN such as Cloudflare) does this resolve the issue? If you are not sure how to clear your server cache you should speak with your host provider.
If clearing the cache doesn’t resolve the issue, we would recommend checking your caching settings and to ensure that the sitemap is being excluded from the cache.
If you are still experiencing an issue with the sitemap not updating after checking your caching settings, you can also force a cache reset by temporarily disabling the sitemap feature, and then re-enabling. To do so please perform the following steps:
In WordPress go to SEO>General>Features
For ‘XML Sitemaps’ Toggle the switch to ‘Disabled.’
Click ‘Save Changes.’
Toggle the switch back to ‘Enabled.’
Click ‘Save Changes.’
We also have a guide on how to enable/disable XML Sitemaps here: https://kb.yoast.com/kb/enable-xml-sitemaps-in-the-wordpress-seo-plugin/
There aren’t “logic” in WP which could redirect sitemap_index.xml from http to https protocol. I quickly checked your sitemaps and it seems that everything works correctly.
It seems that you didn’t create separate HTTPS property on GSC. Other way is to create “Domain property” which includes all variants of you website. On this way, you can set proper HTTPS protocol on GSC and submit only https sitemap.
More about website properties:
https://support.google.com/webmasters/answer/34592?hl=en
So, you need to add https as “URL-prefix property” or update current property to “domain property”.
.htaccess rules for sitemap redirects (if you choice this way – this rule should be before WP rules):
RewriteCond %{HTTPS} !=on
RewriteRule ^(|favicon.ico|robots.txt|[a-z]+-sitemap.xml|sitemap_index.xml)$ https://www.mywebsite.com/$1? [R=301,L]
It redirects favicon.ico and robots.txt too (because they also aren’t handled by WP redirects).
