My online tool can't connect your ipv6 address - https://check-your-website.server-daten.de/?q=2600%3A1700%3Ab310%3Ac2e0%3A%3A2 . If the browser switch to ipv4, that's not relevant.
Do you really use explicit ip addresses? Use <VirtualHost [::]:443>.
PS: Yep, I use telnet to check, if a connection via ipv6 is possible.