Auto-discovered subnet does not get correct VLAN

There is a use case:
- MAAS's DHCP is disabled, and an external DHCP sever is used for PXE boot.
- MAAS has 2 nics, named nic1, nic2. The nic2 is actually the one used for provisioning.
- MAAS based on the nic name order, created fabric-0, fabric-1 for them respectively. The fabric-1 is actually the one used for provisioning, while default fabric is fabric-0 which is almost usless.
- Commission a node, the node get an ip, the node UI shows its IP as "Auto assigned".
- MAAS detected the ip is bound coming from nic2 so it set the node's fabric=1 (more precisely, set vlan_id = id of untagged vlan of fabric-1)
- MAAS found the ip is not belonging to any existing subnets, so it automatically create a subnet, but it always create the subnet on fabric-0. This is the begining of all wrong things later.
- Deploy the node, also succeeded, when finished, it is node UI shows ip correctly
- Release the node, then the node status shows "Node must be attached to a network", this is because it could not find a proper subnet in fabric-1.
- The node become unavailable for Deploying unless Commission again.