The bug:
The atrium in the trial chambers (corridor/atrium_1) is missing a ladder on the left side, from the bottommost block, which is replaced with air.
How it can be reproduced:
In 1.20.6
Make sure Update 1.21 is toggled on!
In 1.21-rc1
No need for any experimental toggles.
Note that it can also be seen in the menu panorama for 1.21-rc1.
Both versions have the same steps to reproduce
1. Use the /locate command to find a trial chambers
2. Teleport to the location given by the command, and then enter spectator mode
3. Go down to the trial chambers atrium room, and observe the location with the ladders.
Observed Result:
The left side has one ladder missing from the bottom
Expected Result:
The left side should match the right side.