I found out the cause, it placed in /var/state/network. If you see more closer, you can notice: it contains a little trash (probably X-Wrt interface can generate it), I commented out it:
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
#config_set '--help' 'ifname' '-help_device'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'wan' 'ifname' 'eth0.1'
config_set 'wan' 'ifname' 'ppp0'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'wan' 'ifname' 'eth0.1'
config_set 'wan' 'ifname' 'ppp0'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
config_set 'loopback' 'ifname' 'lo'
config_set 'loopback' 'up' '1'
config_set 'wan' 'ifname' 'eth0.1'
config_set 'wan' 'ifname' 'ppp0'
config_set 'lan' 'ifname' 'br-lan'
config_set 'lan' 'up' '1'
If you remove this trash, ifup, ifdown etc. will work. Good luck!