When creating a new superflat world, I can using an ender eye to locate to a nearest stronghold or using command /locate stronghold. When arrives, it doesn't generate stronghold, likely "Invisible Stronghold". I know that in superflat world it don't generate any structures except villages. How it works :
1. Make a new superflat world.
2. Throw an ender eye or do /locate stronghold and go to these coordinates.
3. You found no stronghold.

I found this bug accidentally when creating an End Portal in my superflat world. I accidentally throwing ender eye instead of inserted in the end portal frame. When go to these coordinates, there's no stronghold generate.