I was doing the living story achievements with a friend and the secret room accessed by using light dash skill at the end of the Cornered instance was only able to be entered by one person in each instance.
My friend completed the achievement on a group run and the character I was playing on was no longer able to enter the room either in his story instance or my own personal instance after he left group. I believe that my character was flagged as having completed the achievement without actually receiving credit.
The only way I was able to enter the secret room again was to clear the story on a different character and replay Cornered on that character.