Thanks!
Fixed in next update.
You can fix this in the meantime by editing the DangerousTreasures.cs plugin file
Change
if (Physics.Raycast(position, Vector3.down, out hit))
to
if (Physics.Raycast(position, Vector3.down, out hit, position.y, heightMask, QueryTriggerInteraction.Ignore))
I appreciate the reports. The issue here is that it hit a prevent builing Trigger at the monument, and placed the chest there. This will resolve that.