Hmmm Interesting, So when using the Angular package, we use the Angular route events to listen for all different SPA navigations, so it could be related to that.
But for the metrics itself, you can check this document - Supported Technologies | APM Real User Monitoring JavaScript Agent Reference [5.x] | Elastic. All metrics like FCP, LCP, FID measure the load responsiveness and does not really work for route-change events.
Thanks,
Vignesh