IP Address Management
If a dynamic IP address was being used on an Azure resource which then disappeared due to a restart, this IP address is then returned to Microsoft's pool. As a result this particular IP address would not be retrievable, even by Microsoft themselves. Microsoft's official wording on reinstating a previous IP address is below:
So unfortunately this is not a supported scenario, meaning that VM had a dynamic public IP after the maintenance of it, this IP was released and goes back to pool and available to be used by other VMs.
The next step would be to set up a static IP address on the resource and then update all connecting systems with the new static IP address.