I chose the preset 'Desert' customisation for a superflat world, and deleted the sandstone layer. When the game loaded, there were floating mineshafts and a stronghold, along with ground lever spawners. The seed 'lost' is a good example, though it works with almost all worlds. Also, there are sand temples on huge sandstone pillars.
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I have this same bug using super flat, configuration only about 5 blocks high, and mineshaft and others on on 14w8 and 1.7.4
Can no longer reproduce as of 17w48a! 🙂
Probably earlier

i use default superflat has no dungeon,mineshaft but
Strongholds still spawn


Logan Whetzel This bug has been closed and won't be fixed. Please don't add information and screenshots to it.
Where is the bug ?
If you provide the configuration
you'll get mineshafts