In Chrome 89, an improvement was made to the code that reports First Contentful Paint to URL-Keyed Metrics. This change increases the number of reports of First Contentful Paint that Chrome receives right before the user closes a tab. Source code for this change.
We don't see any difference in the values produced by the metric after this change. However, after this change Chrome will now receive more reports of First Contentful Paint, which means that some sites which previously had too little data to be included in the Chrome User Experience Report will now be included, especially on desktop where tab switching is more common.
Chrome 89 is currently scheduled to be released the week of March 2, 2021.