Please help, (going bananaz) trying to trouble shoot sitemap submitted to Bing

[IMSvintagephotos]

We need help to figure out what seems to be an error in our sitemap.
We have submitted the sitemap to BING and the sitemap includes 1,2 million pages that should be crawled.  After initial submission, Bing says in the dashboard that 1,2 million pages have been submitted.  Then always after 2-4 days the number drops to either 500.000 pages or like now 250.000 pages.   Why is that?  is there an error in our sitemap and BING in excluding pages, and it lowers the submitted number after going through them and discovering the error ?.   We need to figure this out and fix so that BING can crawl and index all our 1,2 million pages.   See the screenshot showing the BING dashboard.   
We are also having issues with google but we can't figure out what is going on.

Here are the sitemaps:

https://imsvintagephotos.com/google_sitemap/sitemap.xml

and here:

https://imsvintagephotos.com/sitemap.xml.

Your website is www.imsvintagephotos.com

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[sarahwalsh]

hi if you have a WordPress website, we know that you can use Yoast, which is an seo plugin.

this plugin can automatically create a site map for you.

Hope this helps?

We used it for a company which sells garden bars, to make sure that all the pages get indexed we submitted a site map.