Trying to reach this instance through IPv6 from an IPv6 instance on EC2. Almost there but something around TLS does not seem to be IPv6 ready.
$ curl -6 -H 'Host: mastodon.zunda.ninja' http://dualstack.elbXXXXXX.us-east-1.elb.amazonaws.com/;echo
<html><body>You are being <a href="https://mastodon.zunda.ninja/">redirected</a>.</body></html>
$ curl -k6 -H 'Host: mastodon.zunda.ninja' https://dualstack.elbXXXXXX.us-east-1.elb.amazonaws.com/
curl: (35) Peer reports it experienced an internal error
Using ELB for the default .herokuapp.com domain instead of ELB for Heroku SNI, it seems that TLS access goes through to the app:
$ curl -k6 -H 'Host: mastodon.zunda.ninja' https://dualstack.elbYYYYYY-YYYYYYYYY.us-east-1.elb.amazonaws.com; echo
<html><body>You are being <a href="https://mastodon.zunda.ninja/users/zundan">redirected</a>.</body></html>
An ELB for SSL Endpoint can also be used to accept TLS connections from a v6 address :)