If the peer’s IP address is not in the RIB and is reachable via the default route, then you have to enable ip nht resolve-via-default. This is useful when e.g. you want to allow BGP to peer across the default route which is not so uncommon to find in production deployments.

https://docs.frrouting.org/en/latest/zebra.html#clicmd-ip-nht-resolve-via-default

https://docs.frrouting.org/en/latest/zebra.html#clicmd-ipv6-nht-resolve-via-default