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When you change your VNet settings to point to your customer provided DNS servers on its network interfaces, you must restart VMs for the new settings to be assigned to the VM’s operating system. When the VM reboots it re-acquires its IP address and the new DNS settings are in place.

From

Cover of Exam Ref 70-533 Implementing Microsoft Azure Infrastructure Solutions

Note

When a chance is done to the DNS of the VNET you need to restart all of the VMs of that VNET.