Dynamic Service Fallback allows a service to fall back to a preferred instance when it becomes available after it was failed over to an available instance. This is true because Dynamic Service Fallback is a new feature in Oracle RAC 19c that enables a dynamic database service to automatically relocate to a preferred instance when it is restored, after the service was failed over to an available instance due to the failure of the last preferred instance. This feature improves the availability and performance of the service by minimizing the manual intervention and the number of failovers. To enable Dynamic Service Fallback, you should specify the -failback option for the service as YES when using SRVCTL or DBCA12. Reference:
1: Oracle Database 19c Release 19.3 New Features, Dynamic Services Fallback Option1
2: Automatic Failback of a Service in a Oracle-19c-RAC-Database2