How SQL MI providing the HA?
=> if database and SQL MI in the premium and Business Critical service tier automatically provisioned with the primary read and several secondary read only replicas.
=>The read scale out feature allows you to offload read only workloads using one of the compute capacity of one of the read-only replicas, instead runing on the read-write replica.
in this way premium and Business critical provide performance benefit.
If application connection string configured with the “ApplicationIntent=Readonly the application will be redirected to read-only replica of that database or Managed instance.
you can disable the readscale out feature so that regardless of the applicationintent it will connect to the read-write replica.
Data conistency:
it is possible that latest updates are not immediately visible on the replica. The latency is caused by an Asynchronous transactions log redo operation.
One of the benefits of replica is that the replicas are always in the transactionally consistent, some times there may be latency betwen different replica.
https://docs.microsoft.com/en-us/azure/azure-sql/database/read-scale-out
Azure SQL Database 