if the virtual machine's mac address is in automatic mode, there are some situations where the virtual machine's mac address can change.
AFAIK, this situations are:
- When a virtual machine is powered on again, it might acquire a different MAC address.
- The MAC address changes on the virtual NIC during the hardware (version) upgrade.
- Every time the virtual machine is migrated (vMotion or Storage vMotion)
- When a virtual machine is cloned, the network adapter(s) are given new MAC addresses.