The drop corresponds to this deployment:
14:57 otto@deploy1001: Synchronized wmf-config/InitialiseSettings.php: Migrate QuickSurveys schemas to EventGate on all wikis - T271165, T271166 (duration: 01m 05s)
This change affects navtiming processing of QuickSurveys (https://phabricator.wikimedia.org/T271166#6765813) but also seems like it's affecting processing of all the other schemas the navtiming daemon consumes from Kafka.
NavigationTiming:
SaveTiming: