There seems to be a number of webmasters complaining that the Google Search Console’s hreflang international reporting tool is showing errors and a drop in “known tags” in the report. Webmasters who have implemented them correctly are seeing ‘no return tags’ issues.

@SEO_Suedwest shared a screen shot of the chart showing the drop in known tags:

John Mueller from Google has been responding to folks on Twitter that he is looking into this:

I assume it is just a reporting bug and not a real bug that impacts your indexing and ranking in Google but we will learn more soon.

Forum discussion at Twitter.