Nether generation, ceiling bedrock exposed

Hi,

Since pre relases (first time spoted PRE 3, and still exists) I noticed strange generation in the nether. Ceiling has exposed bedrock. I never saw this thing before as long as I play Minecraft. That happen in multiple palces. No matter of nether biome. 

1) Seed: -1824611495
Example Location: -47 / 106 / -194

2) Seed: -1824611495
Example location: 288 / 90 / -10