Why is `iw` returns 'Not connected.' although wlan connection is established?

Question

I am running a custom yocto build on an embedded platform with a wlan dongle (ASUS USB-N10 NANO B1) and busybox.

I successfully connected the device via wpa_supplicant over wlan to the network. Running commands such as ping or wget results in the expected results. However, when I try to read the SSID or signal strength with /usr/sbin/iw dev wlan0 link 2>/dev/null I get the response "Not connected."

There is only one wlan interface called wlan0:

Output of ifconfig

wlan0     Link encap:Ethernet  HWaddr 7C:10:C9:73:D9:ED
          inet addr:192.168.xxx.xxx  Bcast:192.168.xxx.xxx  Mask:255.255.255.0
          inet6 addr: fe80::7e10:c9ff:xxxx:xxx/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:3445 errors:0 dropped:12 overruns:0 frame:0
          TX packets:160 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:470606 (459.5 KiB)  TX bytes:17278 (16.8 KiB)