printf in Zsh does not shell escape exclamation mark

In Zsh 5.9, we can use printf to shell escape a string:

$ printf '%q' 'One! Two'

One\!\ Two

This produces the correct output of escaping the ! and the space. Now let’s make it as a script:

#!/bin/zsh

printf '%q' "${1}"

Now if we run it, the ! is **not** escaped but the space is:

$ ./my-script 'One! Two'

One!\ Two

If I change the script to /bin/bash (version 3.2), it correctly escapes the ! and space. Is this a Zsh bug, or is there some subtle detail I’m missing?