The address we show here is the address Pi-hole uses when operating in Full IP blocking mode. Since we roll out unspecific IP blocking as standard mode, an empty IPv6 field is of no consequence at all for your Pi-hole.
As you're already quoting your setupVars.conf: The value of IPV6_ADDRESS will be shown in this field. If you want to see this field getting populated you can run pihole -r and select Reconfigure. This will also reconfigure your IP addresses. However, this should not be needed. Just use the 2403:.... address you found using ip a in your router*.
*) Check that this is in fact a static IP. There are many Internet service providers that change the IPv6 prefix on a regular basis. If you find that this is the case, you can go for an ULA based solution. See