This script is setting the right icon for all installed chrome apps in KDE's app panel (Icons-only Task Manager). By default it's only showing the chrome icon which is very confusing.

icons-for-chrome-apps.sh

#!/bin/bash

set -e

DIR="$HOME/.local/share/applications"
PATTERN="chrome-*.desktop"

echo -e "Please backup your $PATTERN files in $DIR first. \nContinue? (y/n)"
read continue_process
if [ "$continue_process" != "y" ]; then
  exit 1
fi

# checks if the xdotool is installed
if ! command -v xdotool > /dev/null; then
  echo "installing xdotool..."
  # installs xdotool
  yes | sudo pacman -S xdotool
fi


for file in $(find $DIR -type f -name $PATTERN)
do
  echo "$file:"
  ID=$(grep -m 1 Exec "$file" | sed -E 's/^(.*)--app-id=(\w*).*$/\2/g')
  CMD=" \&\& xdotool search --sync --classname $ID set_window --class $ID"
  
  # checks if file has been processed already
  if ! grep -q "xdotool search" "$file"; then
    echo "  append xdotool command..."
    # append xdotool command after original command
    sed -E -i 's!^(Exec=)(.*)$!\1\2'"${CMD}"'!g' $file
  fi
  
  
  if grep -q "^StartupWMClass" "$file" && ! grep -q "^#StartupWMClass" "$file"; then
    echo "  disable StartupWMClass..."
    # comment all StartupWMClass occurrecnes
    sed -E -i 's!^(StartupWMClass.*)$!#\1!g' $file 
  fi

done

echo "updating desktop database..."
update-desktop-database $DIR
echo "done"

It somehow also stopped working for me. I figured that the reason (at least for me) was, that there is a StartupWMClass property.
Also, the script is now only considering real files and not symlinks anymore. thx @mdepa91

In general:
This script is changing your *.desktop files and potentially break them. So please make sure you have a backup before starting this script.