As you may already be aware of, Pi-hole embeds a tailored
dnsmasq version that handles DNS and DHCP.
As such, dnsmasq’s documentation is always worth a closer look when dealing with configuration files living in
Quoted from that source:
(…) IPv6 options are specified using the option6: keyword, followed by the option number or option name. The IPv6 option name space is disjoint from the IPv4 option name space. IPv6 addresses in options must be bracketed with square brackets, eg. –dhcp-option=option6:ntp-server,[1234::56] (…)
There’s more, so make sure you read the whole lot at the source.
Also verify you’ve enabled IPv6 in Pi-hole’s DHCP settings.
However, this might not work as you intended:
As IPv6 is strong on auto-configuration, your IPv6 clients may decide autonomously whether they use those settings (click for details).
It is completely at a device’s (or rather, its OS’s) discretion whether it would use Stateful DHCPv6 (about equivalent to IPv4 DHCP), Stateless DHCPv6 or SLAAC to integrate itself into your IPv6 network.
Windows clients (up to Win10, which supports SLAAC as well) will use or at least prefer DHCPv6, while Android devices employ SLAAC.
(As a side note: This is also a common reason for smartphones bypassing Pi-hole when IPv6 is enabled in your network, preferring your router’s (or ISP’s) RA over Pi-hole’s).