This sounds super weird as I can go up and down stairs without an issue and even pressing z at random intervals doesn't cause anything like you describe for me. You're standing on top of the stairs (tile being '<' or '>') and pressing z? Maybe some files didn't download correctly? If you go to your game's directory (I'm not sure where it is on itch launcher I've been using direct download) and go to 'Data > Designs' pretty sure these are the zone map files, any of them that don't end in 0 but end in 1 or -1 are maps that are up or down from the base world Z levels. I would check that those are actually there and not corrupted and filled with nonsense. They should start with like
{
"design": {
"cereal_class_version": 0,
"value0": "Slums",
"value1": 0,
"value2": 0,
"value3": 0,
"value4": [
[
[
1
],
[
1
],
[
and go to something like
],
[
30,
9
],
[
30,
7
],
[
30,
7
],
[
30,
7
],
[
30,
9
],
[
30,
7
],
[
30,
7
],
[
30,
7
]
]
],
"value5": [
[
false,
false,
false,
false,
false,
false,
false,
false,