Suppose you have two Hyper-V servers: On the first server (Server A) you installed both 2018-01 Rollup Update and an updated BIOS release. On the second server (Server B) you installed only the Rollup Update. You added FeatureSettingsOverride, FeatureSettingsOverrideMask, and MinVmVersionForCpuBasedMitigations registry keys to both hosts. Then you rebooted both machines.

On Server A you have a virtual machine. That VM was (re-)booted on that server after the BIOS update and the Rollup Update were installed. Get-SpeculationControlSettings shows that all mitigations are enabled for the machine.
You try to live-migrate the virtual machine from Server A to Server B.

In that case, live migration will complete successfully, but the VM will freeze and won’t be available neither via network, nor via the Hyper-V console.

Solution:

To resume normal VM work, you should either to:

• Move the VM back from Server B to Server A. It should un-freeze automatically.
• Forcefully restart the VM using Hyper-V Management snap-in or PowerShell cmdlets at Server B.

I tested it with Windows Server 2012 R2 only, the VM was Windows Server 2016. Not sure if it applies to Server 2016 hypervisors.