I am seeing the same thing, but wanted to let you know that after the install the correct wifi config is in the /boot/pressed.cfg file if that helps.
osmc@osmc:/boot$ cat preseed.cfg
d-i target/storage string usb
d-i network/interface string wlan
d-i network/auto boolean false
d-i network/ip string 10.0.0.160
d-i network/mask string 255.255.255.0
d-i network/dns1 string 8.8.8.8
d-i network/dns2 string 8.8.4.4
d-i network/gw string 10.0.0.1
d-i network/ssid string xxxxxxxxxxxxxxx
d-i network/wlan_keytype string 1
d-i network/wlan_key string xxxxxxxxxxxxxxxx
osmc@osmc:/boot$