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You can restore a database from one instance of Azure SQL Managed Instance to another only if two instances are in the same subscription and region. A cross-region and cross-subscription restore aren't currently supported.
You cannot restore all Azure SQL Managed Instance by using the point-in-time restore. You can restore only a database that is hosted on Azure SQL Managed Instance.
You need to be aware of the storage size of your SQL Managed Instance. Depending on size of the data to be restored, you might run out of instance storage.
You can use a point-in-time restore capability for a new, restored, or copied database from the time when the initial transaction log backup that follows the initial full backup is created. The first full backup is scheduled immediately after a new database is created or restored. This backup usually completes within 30 minutes, but it can take longer when the database is large. For example, the initial backup can take longer on a restored database or a database copy. After the first full backup, all further backups are scheduled and managed automatically. The exact timing of all database backups is determined by the Azure SQL Managed Instance service as it balances the overall system workload. You cannot change the schedule of backup jobs or disable them.
See also: The ephemeralConnectionRetryTimeout property for Azure SQL Managed Instance