Wrong icon after application startup

I have two Eclipse installations and for each of them a desktop file, see below.

For the icons, I'm using eclipse in both cases with an activated Numix Icon Theme.

If I search and start the applications via Activities, the icons are correct.

But not in the taskbar on the left hand side after the start

How can I change this behaviour, I want the Numix icons on the left hand side also.

Some more details

find /usr/share/icons/Numix-Circle -iname "*eclipse*"

/usr/share/icons/Numix-Circle/48x48/apps/redeclipse.svg
/usr/share/icons/Numix-Circle/48x48/apps/eclipse.svg

cat ~/.local/share/applications/opt_eclipse_cpp.desktop

[Desktop Entry]
Encoding=UTF-8
Version=1.0
Type=Application
Name=Eclipse C++
Comment=Eclipse Integrated Development Environment
Icon=eclipse
Exec=/opt/eclipse-cpp/eclipse/eclipse
StartupNotify=true
StartupWMClass=Eclipse-CPP

cat ~/.local/share/applications/opt_eclipse.desktop

[Desktop Entry]
Encoding=UTF-8
Version=1.0
Type=Application
Name=Eclipse JEE
Comment=Eclipse Integrated Development Environment
Icon=eclipse
Exec=/opt/eclipse-jee/eclipse/eclipse
StartupNotify=true
StartupWMClass=Eclipse-Java