Explanation
https://learn.microsoft.com/en-us/azure/site-recovery/azure-to-azure-network-mapping The subnet of the target VM is selected based on the name of the subnet of the source VM.
- If a subnet with the same name as the source VM subnet is available in the target network, that subnet is set for the target VM.
- If a subnet with the same name doesn't exist in the target network, the first subnet in the alphabetical order is set as the target subnet.