Service router subset filters

Consul
1

I am trying to create a service resolver that will matches az3 if the condition

filter = "Service.Meta.instance_id == Node.Meta.instance_id"

is met, see below for full HCL file:

kind = "service-resolver"
name = "web"

default_subset = "az1"

subsets = {
  az1 = {
    filter = "Service.Meta.az == 2a"
  }
  az2 = {
    filter = "Service.Meta.az == 2b"
  }
  az3 = {
    filter = "Service.Meta.instance_id == Node.Meta.instance_id"
  }
}

However this does not appear to work. As per the documentation Filtering | Consul by HashiCorp, Values can also be Selectors.

Interestingly if I do the following:

filter = "Service.Meta.instance_id == `i-123456`"

or

filter = "Node.Meta.instance_id == `i-123456`"

it resolves healthy instances of the “web” service.

Any ideas what I might be doing wrong?