Using CloudFormation, I cannot get an AAAA record to work with ELB

I have the following two CloudFormation resources:

"TestELB" { ... },
"TestRecordSetGroup": {
  "Type": "AWS::Route53::RecordSetGroup",
  "Properties": {
    "HostedZoneName": "example.com.",
    "RecordSets": [
      {
        "Name": "subdomain.example.com.",
        "Type": "A",
        "AliasTarget": {
          "HostedZoneId": {"Fn::GetAtt": ["TestELB", "CanonicalHostedZoneNameID"]},
          "DNSName": {"Fn::GetAtt": ["TestELB", "CanonicalHostedZoneName"]}
        }
      },
      {
        "Name": "subdomain.example.com.",
        "Type": "AAAA",
        "AliasTarget": {
          "HostedZoneId": {"Fn::GetAtt": ["TestELB", "CanonicalHostedZoneNameID"]},
          "DNSName": {"Fn::Join": [".", ["ipv6", {"Fn::GetAtt": ["TestELB", "CanonicalHostedZoneName"]}]]}
        }
      }
    ]
  }
}

After the stack updates, I see both records listed in my zone with the expected alias values. The A record works, as verified with dig:

$ dig A subdomain.example.com
...
;; QUESTION SECTION:
;subdomain.example.com.       IN      A
;; ANSWER SECTION:
subdomain.example.com. 59     IN      A       11.22.33.44
;; Query time: 38 msec
...

But the AAAA record does not work:

$ dig AAAA subdomain.example.com
...
;; QUESTION SECTION:
;subdomain.example.com.       IN      AAAA
;; AUTHORITY SECTION:
example.com.         899     IN      SOA     ns-1234.awsdns-11.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
;; Query time: 54 msec
...

I think it has something to do with Fn::Join being used to add ipv6. to the beginning of the ELB's DNS name. If I change the A record so it uses Fn:Join to prepend dualstack. to the DNS name it also fails in the same way.

Is Fn::Join the correct way to add ipv6. or dualstack. to the beginning of a DNS name?