From the RDS documentation:
You can stop and start a DB instance whether it is configured for a single Availability Zone or for Multi-AZ, for database engines that support Multi-AZ deployments. You can't stop an Amazon RDS for SQL Server DB instance in a Multi-AZ configuration.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_StopInstance.html#USER_StopInstance.Limitations