What would cause a sudden drop in indexed sitemap pages?

[EcommerceSite]

I have made no changes to my site for awhile and on 7/14 I had a 20% drop in indexed pages from the sitemap. However my total indexed pages has stayed the same. What would cause that?

[David-Kley]

I know that the upcoming Panda refresh has been a popular topic in many SEO forums. Lot's of people saying the same things: Noticing a sudden drop or increase in indexed pages. I will see if I can dig up any info that would help. Most of what I have read has been speculation, so I'd rather not share something that could be inaccurate.

[PatrickDelehanty]

Hi there

It appears Google had a bug. You can read more about it here.

TL;DR - "This is not uncommon for the Google Search Console, formerly Google Webmaster Tools, reporting interfaces to have glitches. But this one is one of the more serious ones...If you see your page count drop like this in Google Sitemaps reporting, you are not alone, Google is working on fixing it." Hope this helps! Good luck!

[EcommerceSite]

This was just put out https://www.seroundtable.com/google-sitemap-index-count-drop-is-a-bug-20608.html