There is a long-established inconsistency in BGP options. Options for IPv4 are right inside the neighbor, while IPv6 options are inside "address-family ipv6-unicast". For example, we have "neighbor x.x.x.x route-map export Foo" for IPv4, but "neighbor x.x.x.x address-family ipv6-unicast route-map export Foo" for IPv6.
FRR migration uncovered a nasty shortcoming of this: its correct functioning was dependent on particular behaviour of the old Quagga that enabled IPv4 on all peers with IPv4 addresses, whether "no default ipv4 unicast" option was set or not. FRR exhibits logical behaviour and disabled IPv4 BGP on all peers if that option is enabled.
Making IPv4 and IPv6 syntax consistent will not only improve, well, syntax consistency, but also automatically resolve the issue with that behaviour change.