This type of prefix length would be suitable for a single address such as a loopback interface or an IPv6-capable host. We use /32 prefix lengths in IPv4 to do similar things like Route Health ...

The prefix length for classless IPv4 addressing must be given (by the netmask) to properly place the boundary between NetID and HostID portions of the IPv4 address. IP addresses, both IPv4 and IPv6, ...

You specified a specific /64 prefix for the ipv6 address you statically assigned to the router, but the local subnet prefix you gave to nd is a completely different /64.

Current recommendations say that each subscriber to an IPv6 network should be assigned an entire /48 prefix. This means you have 16 bits to use for further subnetting and then 64 bits still left ...