/locate command does not locate certain structures in Superflat

The bug

The locate command always returns "Could not find that structure nearby" even when the structure is right below you.

Affected structures

• Ocean monuments
• Ocean ruins
• Shipwrecks
• Igloos
• Desert pyramids
• Jungle temples
• Witch huts

How to reproduce

1. Enter singleplayer
2. Create superflat world with preset:

   minecraft:bedrock,38*minecraft:stone,5*minecraft:dirt,5*minecraft:sand,110*minecraft:water;minecraft:deep_ocean;oceanmonument,decoration,stronghold,mineshaft,dungeon
   

3. Enter /locate Monument

Code analysis

Code analysis by liach can be found in this comment.