I've done some exploring a few minutes ago(mainly to see how the advancements progress), and went into the Stronghold. Basically half of the stronghold is a combination of a ravine + a mineshaft + the stronghold itself. I'm not sure whether this is a bug or just the seed itself, though it seems most likely to be the first. Here are a couple screenshots.
Environment
I'm running the latest snapshot(17w17b) on Windows 7. No mods installed, full vanilla. PC has 6 GB of RAM, no problems running Minecraft. I'm pretty sure I'm using the latest Java version.
I noticed this issue, not fixed as of 1.12.2