VM's disappear from LAB hosts. those were cloned(or)imported from PROD (V2V) (and also a part of the same VMM).
I didn't made those clones\imports so I'm not sure, but maybe the GUID's are the same as the VM's from the PROD, so maybe that's what cause the VM's to disappear.
after a machine disappears under LAB host group, something strange happens: I see for few milliseconds that a machine which usually under PROD host group (with the same name as the one that disappeared) shows under this host group, that's what lead me to think that a duplicate GUID might cause that.
I haven't found anything online.
p.s - when I check those machines from the Hyper-V Manager - all the machines exists and kicking.
any help \ suggestions would be much appreciated.