Regression - unable to select proper subnet when adding interface alias

[Heather Lemon]

The selection of default VLAN and Fabric has changed in the Add Alias drop down from 2.9 to 3.0.

Adding an alias to the correct interface, from the drop down works in MAAS2.9, and does not work in MAAS3.0
(see attached screenshot)

Steps to reproduce:
1. Deploy MAAS 3.0.0 (3.0.0-10029-g.986ea3e45) via deb or snap
2. Add VLAN with two subnets (IPv4 and IPv6)
3. Connect machine interface to VLAN
4. Add an alias to the interface
5. Default subnet will be used
6. Edit alias to assign correct subnet/vlan

Ubuntu series - focal
Tested with both MAAS 3.0/stable: 3.0.0-10029-g.986ea3e45
and 2.9/stable: 2.9.2-9165-g.c3e7848d1

[Heather Lemon (hypothetical-lemon)]

MAAS2.9 screenshot

[Christian Grabowski (cgrabowski)]

Hmm both screenshots show the VLAN (and fabric) dropdown(s) as disabled. That aside, are you able to create an alias correctly via `maas $PROFILE interface link-subnet $SYSTEM_ID $INTERFACE_ID [ARGS]` (the CLI Equivalence of what the UI should be doing)?

[Heather Lemon (hypothetical-lemon)]

I have been unsuccessful in adding an alias via the CLI.

I also noticed that adding the alias through the UI works when the disabled vlan and fabric info are incorrect during adding, but afterwards it is applied/saved to the correct interface.

I attached a screenshot of adding the ipv6 alias to the fabric-B

[Heather Lemon (hypothetical-lemon)]

adding ipv6 alias to vlan-200 fabric-b works but during the addition of adding the alias the disabled vlan, and fabric names are incorrect

[Heather Lemon (hypothetical-lemon)]

the add a new alias is disabled, which is the correct behavior, but the wrong information is being displayed

[Heather Lemon (hypothetical-lemon)]

This is just an issue where the wrong info is being displayed in the drop down thats greyed out