During the treasure hunt event, you are not able to interact with any of the treasure you find using the metal detector.
No prompt is displayed to pick up the treasure - it’s as if you are too far away to interact with it. Swapping size potions and switching player models seems to have no effect.
Edit: After trying again, I have found that most of the time your metal detector won’t get within 150cm of any treasure. It’s as if the entire map was moved up but the spawn points for the treasure weren’t changed to reflect the new height.