Wait, there is some confusion going on here. The setting on the Pi-hole UI is not about rapid commits for DHCPv6 (which is altogether a separte beast!). It is only about rapid commits for IPv4 DHCP.
Instead, enabling this feature does the following according to the documentation (emphasis added):
Enable DHCPv4 Rapid Commit Option specified in RFC 4039. When enabled, dnsmasq will respond to a DHCPDISCOVER message including a Rapid Commit option with a DHCPACK including a Rapid Commit option and fully committed address and configuration information.
Should only be enabled if either the server is the only server for the subnet, or multiple servers are present and they each commit a binding for all clients.
DHCP rapid commit is always a good idea when the requirements above (bold text) are fulfilled. This is basically always the case at home but basically means not always, hence it is disabled by default. DHCPv6 is more experimental and can help with some misbehaving routers. However, I'm not recommending it. But if it can solve an IPv6 issue it's fine to be there.