I'd put my money on a fall-through error condition where TSC is simply the last one
with a 'good' error message pointed to. I have clusters with CPUs that are both 10
years and 10x apart in performance performing migrations between themselves quite happily
(Sandy Bridge dual quads to Skylake 56 cores), as long as you make sure the cluster and
default machine type low enough.
Ok this is 4.3 still, but...
So what if you start the VM on the 'weak' host first? Can it then move freely?