Prior to Chrome 86, there was a bug in Largest Contentful Paint on some pages where the largest contentful element initially has opacity:0. The bug was fixed in Chrome 86. The source code of the change can be seen here.
This change only affects metrics for a very small amount of sites. Generally sites whose largest contentful elements are opacity:0 initially are using A/B testing frameworks which initially clear the page before displaying the correct content. Sites using this technique will now see a longer LCP, reflecting the time until the largest contentful paint is visible to the user, instead of the time it is loaded in the DOM but invisible.
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 86 the week of October 5, 2020.
