This setting helps to keep the netmask to be /32 (255.255.255.255) of the loopback alias on reboot.
Use CIDR notation specifying /32 in /etc/hostname.lo0:1, e.g.:
# cat /etc/hostname.lo0:1 10.1.1.1/32
Use also CIDR notation for all normal interfaces specifying the normal netmask, e.g.:
# cat /etc/hostname.bge0 10.1.1.10/24
And last but not least remove the network entry and corresponding netmask from /etc/netmasks.