Ok, I publish the save.
The link for play with it : https://powdertoy.co.uk/Browse/View.html?ID=964524
The walls are still there, just not visible in the images rendered on the website, and will disappear if the save is loaded in TPT version 83.0. They load fine in later versions.
Powder Toy versions later than v83.0 are the C++ rewrite, TPT++, which saves walls in a different format. Version 83.0 and the save renderer on the website are the original C version of Powder Toy, which doesn't understand the new way of saving walls.