BIRD put a malformed BGP message on the wire
From what I'm seeing here, I would agree. Ideally, someone should
report this to the BIRD developers. That said, for such a bug
report to be useful, we may need more information. If it was just
the RFC 1918 next-hop that triggered this, then this should be
trivially reproducible. But if it was something else, we may not
have enough information to get to the bottom of this.
-- Richard