Gold Skull Cape question

Killed Nomad under 6 mins on a non-reward run, wiki says I’m still eligible for cape, didn’t use a dart or anything not allowed and my time was counted, but I didn’t get any cape and my achievements say I still haven’t got a kill under 16 minutes. How do I claim the cape / have I done something wrong for it not to count? I can’t see any reason why it wouldn’t.