The Ir Yut phase with shriekers not spawning. Shriekers do not spawn until the deathsong starts. It's possible to kill all the adds/mobs upstairs along with the 2 wizards without starting the deathsong, so running in gives the illusion of a glitch due to the lack of shriekers. To make them appear, simply attack the mobs down below and the deathsong will start, spawning in the shriekers.