It seems that the adapter will accept only locally administered MAC addresses as valid (makes sense). This is signified by 7th leftmost bit set to 1, so MAC must start with 02. I changed it without any problems:
Ethernet adapter Local Area Connection 3:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft Loopback Adapter
Physical Address. . . . . . . . . : 02-FA-FA-FA-FA-FA
Dhcp Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 10.0.0.1
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . :
If you try to define it as Globally unique (00
), it treats it as invalid and reverts to default:
Ethernet adapter Local Area Connection 3:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft Loopback Adapter
Physical Address. . . . . . . . . : 02-00-4C-4F-4F-50
Dhcp Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 10.0.0.1
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . :