I'm having this issue as well. I have noticed the loose flag stone in many different locations also, I have collected the loot from it at least 5 times from various maps. Before I even knew it was part of this quest. I could not complete the quest the way I wanted. I had to bully the poor kid into telling me what the secret was.
Playing on Xbox Games pass version as well.
I can share a game file. It says it is to big to upload though. 2,075kb