[2.1, UI] Broken validation on VLAN MTU

Actually in this case the validation is not wrong. There is an issue with your configuration as MAAS now believed that your rack controller cannot control that VLAN. I would assume that means your rack interface was changed to a different interface while controlling that VLAN.

There is a bigger issue here really. When a rack controller is the primary or secondary controller for a VLAN its interfaces should not be allowed to be changed where that rack controller would be disconnected from the VLAN it is controlling, either by user action or automatically. The automatic case is the hardest when something like that occurs we need to perform some action to alert the user that on of the rack controllers that should be running this VLAN no longer can, so either MAAS automatically picks a new rack controller that can manage the VLAN or we disable DHCP and alert the user.

I have only one VLAN: the default untagged VLAN. I don't think I've done anything particularly weird with my configuration. I now have a region+rack controller and a rack controller. Each is on a NUC, with one network interface per NUC, on an isolated network on which there's one other NUC and my laptop. When I go to that VLAN page, put the focus in the MTU field and tab out, I get two errors:

    primary_rack: Select a valid choice. 1 is not one of the available choices.
    secondary_rack: Select a valid choice. 15 is not one of the available choices.

Sometimes I get four errors: the same as above, but repeated.

Hmm. I would say the code that is calculation what is available choice could be incorrect. Check all your rack interfaces make sure they are all on the same fabric/vlan combination.