The author means that there won't be a negative amount of passengers, so WE don't need to error check for his tests.

In the problem condition the author suggests that passanger rest in the bus ALWAIS (for any bus stop) great or equal to zero. But tests don't check this!

I don't undestand:
arrays = [[-8, -8], [0, -6], [4, -8], [2, -4], [-2, 4], [-1, -5], [7, -2], [-5, 4]]
shift = 1
my result: [-8, -8, -2, -6, -6, 3, 2, -7, 4]
estimated result: [-8, -8, 0, -6, 4, -8, 2, -4, -2, 4, -1, -5, 7, -2, -5, 4]

Manually:
-8 -8
0 -6
4 -8
2 -4
-2 4
-1 -5
7 -2
-5 4
-8 -8 -2 -6 -6 3 2 -7 4 # as in my solution

Why estimated result is -8, -8, 0, -6, 4, -8, 2, -4, -2, 4, -1, -5, 7, -2, -5, 4 ???
Such result is correct for shift = 3

You forgot that ascii-code contains some symbols wich ascii-number less than ascii-code for digits, for example '#@%'. So, if input string will contain words with one of those symbols, your solution will work wrong.
Try this: "A9# fg1 ghji3%"

This code don't pass the test below:
[[2,4], [2,6], [2, 8]] -> [[6, 12], [4, 12], [3, 12]]
See kata condition for Bash