Actually it works. The UDP6 port is being listened to. The 'netstat' command I used is from BusyBox and it shows UDP6 as
udp 0 0 :::161 :::*
I looked for the first token of 'udp6' and thought the UDP6 port was not listened. It's my mistake while seeing the display of BusyBox's netstat.
So, the both UDP port 161 of IPv4 and IPv6 are being listened.