It seems that’s consequence because you have forced indexing of sitemap_index.xml. You should only submit sitemap_index.xml as “sitemap” (not as other pages). Sitemaps use XML (made for machines and it isn’t HTML). These files aren’t designed to be “mobile friendly”.

Regarding that there is “noindex robots tag”, this URL will be automatically removed from google index (probably in next few weeks). If you don’t make new mistakes, it will permanently removed from google index (and warning about mobile friendly will be removed), but googlebot will continue to use these XML to improve indexing.
Thank you for your reply.
I didn’t intend to force indexing the sitemap but somehow it ended up like that and I don’t know how that happened. The errors showed up all of the sudden. What steps should I take to fix them?
I once deleted it and resubmitted right away. Maybe I should wait a day or two then resubmit? would that help? or should I wait until Google removes it in next few weeks?
I think that sitemaps are submitted correctly. Anyway, you could confirm it on GSC -> Index coverage (Google Search Console). It’s better to wait some time (couple weeks).
I see correct sitemaps (and their HTTP headers) because you have excluded the sitemap from caching. It’s possible that Hummingbird doesn’t cache properly HTTP headers for sitemaps (Content-Type and X-Robots-Tag). I know that were fixed in WP Super Cache and WP Rocket. It’s possible that googlebot was crawled the sitemap as “HTML” (if Hummingbird didn’t set proper Content-Type. It could explain this issue.
This thread has been marked as resolved due to a lack of activity. You’re always welcome to re-open this topic.
Thanks for understanding!