When a time series stops uploading, the chromeperf dashboard doesn't notice or flag it or try to explain it. If a user is manually monitoring the time series, then they may file a bug like 863813, but usually the data stoppage goes entirely unremarked. The dashboard used to have a StoppageAlert system that required sheriffs to check that stoppages were expected, but it was removed because they almost always were. However, there are still occasionally unintended stoppages. Most of these stoppages can be explained by the flakiness dashboard and/or sheriff-o-matic.
The chromeperf dashboard should visually flag stale charts and try to explain why there hasn't been new data recently using signals from flakiness/sheriff-o-matic.