R1 is a route reflector client, and the prefix 2001:db8::1/128 is originated by a peer with the router ID of 3.3.3.3. Both R1 and the router with ID 3.3.3.3 are in the same AS, meaning they can share routes internally via the BGP Route Reflector (RR). The summarization must occur at the origin of the prefix (3.3.3.3) since that router is responsible for originating the more specific prefix (2001:db8::1/128).
To resolve, configure an aggregate address on the router with router ID 3.3.3.3 to summarize the specific prefix (2001:db8::1/128) into a broader /64 prefix:
router bgp <AS>
address-family ipv6
aggregate-address 2001:db8::/64 summary-only
This ensures that the summarized /64 prefix is advertised within the AS and to external peers while suppressing more specific routes.