Google has confirmed on Twitter that there may be a sitemap index count reporting bug within the Google Search Console.
Gary Illyes from Google said on Twitter, “We might have a problem with reporting the number of indexed URLs in the Search Console Sitemaps feature.” The “good news,” Illyes added, is that “we’re looking into it.”
I checked the webmaster forums and chatter in the community and honestly did not see widespread reports or concerns with index count drops. This is unlike when we had this issue a month ago, when there was panic in the community as webmasters noticed huge drops in their index count.
I did find one webmaster who confirmed a drop in index counts. Maybe webmasters will start noticing the bug today or tomorrow, but hopefully, the issue will be resolved before there is any major concern.