Workspaces Selector Webapps display webrowser icon instead

I have created some webapps based on firefox and chrome using *.desktop files.
Webapps work fine.

At first the the Plank dock was using wrong icon once the webapps where active (the "?" icon). I solved it adding "StartupWMClass=Navigator" to the *.desktop files.

However the Workspaces Selector on my panel is using the webrowser icon, be it firefox or chrome.

Would there be a way to fix the icon display?
Do you think this is related to the workspaces selector?
or to the *.desktop file?

I attach an image showing how Plank has correct icons highlihgted while workspaces selector shows all as firefox icons.

thank you