I managed to fix my code with the help of the first comment, the fix looks like this:
`!for file in data/kapitel*.txt;

do dirname="${file%.*}";

printf 'Creating directory: %s\n' "$dirname" >&2;

mkdir -p "$dirname" && awk -v dir="$dirname" '/^Artikel[[:space:]]/ { close(f);

f = sprintf("%s/Artikel%d.txt", dir, ++n);

printf "Writing to file: %s\n", f | "cat>&2"};

f {print > f}' "$file";done

This is technically one line in my code, I seperated it for some readability. Issue seems to have been

1. Bad use of

   !ls data/kapitel*.txt \
   | while read file; do
   

   which was replaced by the first line of code
   2. messed up combination of awk and bash