According to RFC 2545 (https://tools.ietf.org/html/rfc2545
) a BGP update can include the Link Local Address next to the global address is they share a common subnet.
Quote from the RFC
The link-local address shall be included in the Next Hop field if and
only if the BGP speaker shares a common subnet with the entity
identified by the global IPv6 address carried in the Network Address
of Next Hop field and the peer the route is being advertised to.