Eyes of Ender point to stairs after reloading a world

Every time I use an Eye of Ender, it doesn't seem to lead to the Stronghold's Portal Room. Instead, it leads to a spiral staircase room in the Stronghold. Even if the End Portal is just 5 blocks away, it leads AWAY from the portal.

Steps to reproduce:

• Create a world with the seed

  MC-70143

• Throw ender eyes, those lead to 640 / 36 / 688
• Leave the world
• Reload the world
• Throw ender eyes, those lead now to 645 / 40 / 645