As reward for killing Patchwork Juggernaut under 4 minutes, you should get the Mask of the Sleepers, brown. I did it and get an achievement. What I didnt get is, howewer, reward.
It looks like something has broken in the awarding of this particular clothing item. We’re looking into a fix. When fixed, we will make sure everyone who has already earned the achievement will get the clothing at that time.
Sorry for the inconvenience, and thank you for reporting this!