When IP address is being added to an interface, a kernel installs loopback route.
Such route to local address created with ifa_maintain_loopback_route() function in https://svnweb.freebsd.org/base/head/sys/net/if.c?annotate=323170#l1711 misess RTF_PINNED flag, so that it cannot override possibly existing route to that address added
by a routing daemon. EEXIST is returned in such case.
This error is then propagated back to userland ioctl SIOCAIFADD breaking
interface configuration sequence. See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=223129
for real world example.