You cannot define a DHCP reservation for the same MAC address for a device connected to multiple VLANs
Additionally if, for example, you do setup a machine with a untagged and tagged VLAN, it receives DHCP addresses for each network, but in the client list of the controller it only shows one entry for client, when there should be two.
This is a real mess in the UI and needs sorting out.
If the system can serve DHCP addresses to the same MAC across multiple networks (which it can), then there needs to be:
a) A mechanism for reserving these addresses
b) Each client displayed correctly in the client list
At the moment it looks like the system is treating the MAC as a unique identifier, when it should be MAC+Network.
This is actually a big problem, and may be related to existing problems with inconsistent handelling of DHCP.