In order to reproduce:

Create buffet with any ocean biome selected with any surface, caves, or floating islands.

Or

Create superflat with "water world" preset selected

Or

Create superflat with this preset code

minecraft:bedrock,5*minecraft:stone,5*minecraft:dirt,5*minecraft:sand,90*minecraft:water;minecraft:deep_ocean;stronghold(count=7),oceanmonument,biome_1

Then

/locate Stronghold 

will not find any.