The Assembly trial chamber has an upper entrance that has "staircases" leading up to it from either side. However, sometimes these pieces fail to generate because dispensers (generated earlier) are in the way.

How to reproduce

1. Open a world with the experimental 1.21 datapack enabled and seed 4226348645603381645.
2. /execute in minecraft:overworld run tp @s -24.41 -22.78 -305.17 -768.60 36.75
3. There is an empty space where a staircase should be:
   

Cause
The dispensers that can generate the the bottom of the staircases block the staircases from generating, if the dispensers are generated first. These staircases have a horizontal size of 11x6, so they do not fit if the dispensers are already generated. The following image from my own jigsaw previewing tool illustrates the issue:

!
The orange bounding box of the dispenser (bottom left) intersects with the white bounding box of the attempted staircase placement, causing the failed placement.