Hmmm I didn't add anything specific for IPv6, could be something related to Heroku (where my app is hosted). Can you share an IPv6-only host with me so that I can run some tests?

I've not used Heroku, but likely it's because they don't give you IPv6 in the thing that does the checking.

Try https://ipv6.google.com/

On your page I get:

    connect ENETUNREACH 2607:f8b0:4004:c08::8a:443 - Local (:::0)