Google said it is fixing the issue and there is no action you need to take.
It is believed that this started sometime around September 22 or 23. That’s when the SEO community began noticing big changes to the search results.
Google’s confirmation. Google said on Twitter via the Search Liaison account, “we are currently working to resolve two separate indexing issues that have impacted some URLs. One is with mobile-indexing. The other is with canonicalization, how we detect and handle duplicate content. In either of these two cases, pages might not be indexed.”
Mobile-indexing issue. Google explained that in some cases it was not indexing pages “if a previously indexed page has gone, it might be the mobile-indexing issue, where we’re failing to select any page at all to index.”
Canonicalization issue. Google also added that it may not index pages “if the canonical issue is involved, URL Inspector may show the URL as a duplicate & the Google-selected canonical will be different from it.” I posted numerous examples of this over here on the Search Engine Roundtable and Google even asked for examples earlier this week.
In short, it looked like in some cases, Google was showing a completely wrong URL for a page or not showing that page at all in its index.
What can you do? Nothing. Google said it will take care of the issue. Google wrote “there’s no action to take with these issues on the part of site owners. So wait it out and hopefully Google will resolve it soon. But this has been lingering on for the past 8-days or so. “We apologize for the issues here and are working rapidly to resolve them. We’ll update this thread as each is corrected,” Google said.
The thread: Here is the thread on Twitter:
Why we care. Many of you have been scratching your heads since September 23rd around these changes in Google search. Now you know, it may have just been a bug with Google and your rankings and traffic will hopefully return when Google resolves the issue.
It can be that your site was impacted by something else and your rankings will not return but at least you know there was some sort of bug on Google’s end around that date.
Postscript. Google has given us an update on these issues. Google said “it may take days to fully resolve both these issues completely, but we have restored many URLs already and are working quickly to process more.”
Google gave us some figures, specifically that the issue with canonicals impacted roughly about 0.02% of the Google index, beginning around September 20th, which is earlier than we expected. It lasted until late October 1st around 4:30pm PT. Google said it has since restored about 10% of those URLs and keep reprocessing more.
With the mobile-indexing issue, Google said it impacted roughly about 0.2% of its index, beginning in early September “but really spiking from around the middle of this week through late yesterday,” Google said. Google said it has since restored about 1/4 of those URLs & keep reprocessing more.
Postscript 2. Google posted another progress update on this fix, saying “we’ve now restored about 25% of the URLs impacted by the canonical issue and about 50% of those impacted by the mobile-indexing issue. We continue to keep reprocessing more.”