To reproduce this:

1. Create a world with the seed '2909526293518662168'.
2. Teleport to 1857 60 1891
3. See there is a large ocean ruin.
4. Dig up and open a few of the outer chests, and see that most of their loot is the exact same. (6 are the same, 2 are the same of different loot)

This kinda makes the loot in the larger ruins very lack-luster, as it's all the same!