I did read your initial post, I'd suggest if you're unsure how my suggestion relates to your issue you ask for clarification. As I stated previously, The only thing close for IPv6 that will do this is https://documentation.cpanel.net/display/DD/WHM+API+1+Functions+-+ipv6_range_list but it doesn't return the domain, it returns the cPanel username since IP's are assigned to a user and not a specific domain.
This is why I in turn suggested you use the DIG command.
Please let me know if you need further clarification on these statements.
Again, I am telling you the only API function that will do this is the one I've listed which pulls the IPv6 range list assigned to an account as cPanel does not assign IP's to domains. Otherwise, there is not an API function that will give you the IPv6 address assigned to a domain. Which was your initial question: