Are you sure you fully completed it? The honeycomb has to be at 100%, and you have to be standing in the white square behind it while the vinewrath does its laser beam; if you get knocked out of the box for each pass, it won’t count.
As for the “Challenge completed”, that’s just misleading text that’s there whether you’ve completed it or not. My first picture is from my account that hasn’t done that achievement, and the second is from my main account (that has done it).
