Dmitry, An RS with a unicast destination is legal as per RFC 4861. For example, first time when a host was initialized, the host sent an RS with mcast destination. Then the host received an RA and the host acquired IPv6 address(es) and is up and running. After a while the host detaches from the link and reattaches, or the host network interface is disabled by admin and enabled again. In such cases, the host does know the unicast destination of the default router. So the next time the host is enabled or reattached, the host can unicast the RS directly to the router.
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