The bug
The structure underwater_ruin/mossy_1 does not have a chest, although all other underwater ruins have one.

To reproduce:  MC-278851 mossy_1.mp4

1. Place the structure:

   /place template minecraft:underwater_ruin/mossy_1

   check if any of its structure blocks spawn a chest

1. Place similar structures:

   /place template minecraft:underwater_ruin/brick_1
   /place template minecraft:underwater_ruin/cracked_1

   check if any of its structure blocks spawn a chest

What I expected to happen was...:
See all structures with chests

What actually happened was...:
The structure mossy_1 had no chest