Yes, this is expected behaviour and has been implemented since 8.1.
It cannot be turned off as it is for security to make sure you cannot plug any device into a dock and get on the network, if you're using MAC based security. Allowing you to turn it off would put a back door into this.
You should register the virtual MAC addresses on the devices rather than the dock MAC address and then you know it is the MACHINE and not the DOCK that has the IP address.
Wim
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