Seen this "problem" now a couple of times. For sure the meltdown/spectre bios patches together with updated EVC modes are the trigger of the problem. We always fix it by creating a new cluster and moving the hosts into it followed by most of the VMs by performing a vMotion. At the end we often have a couple of VMs which cant be vMotion'd and we have to shutdown these VMs to move them.
IIRC, there is something in the release notes when the vCenter comes with the updated EVC modes.
Regards,
Joerg