Can't get chest in Crossroads... LAST CHEST I NEED FOR 100%! I cri now...

Hello, 

I've pretty much wrapped up everything and I am on the final chest to 100% the entire map. However, it seems this chest out of all 140 is the only one locked *inside* a quest. I've looked high and low and I can't find a way to get to it. 

I have googled it and they're all saying it's part of the quest "The Labs Below" . Surely this is some oversight as 139 chests out of 140 are all accessible outside the quests. 

Could we get a fix please? As I want to complete the game but this is going to send me to the moon.