Prior to Chrome 81, the Largest Contentful Paint (LCP) reporting logic for the Chrome User Experience Report had a bug: if the largest image on the page was still loading, but the largest contentful paint on the page was text, the largest contentful paint would be recorded as the text. As of M81, it will be recorded as the largest image, unless the largest image does not finish loading before user input or the page is hidden, in which case LCP will not be recorded. The source code of the change can be seen here.
Sites with very slow loading images which are smaller than the largest text block on the page will have additional LCP measures which would not have been previously reported. Generally these are slower-loading pages, so the site's overall metrics may shift higher.
We do not see an impact from this change in our overall metrics, so we believe the effect on most sites will be minimal.
Most users were updated to Chrome 81 the week of April 20.