Missing applications taskbar icon

Some applications are missing the taskbar icon.

I'm using Ubuntu 18.04.3 LTS - Mate - 64 Bits.

Where the applications icons are configured to be showned in taskbar?

Is there a place on file system? Is it configured by application code

Some threads on ask ubuntu say to configure a variable named StartupWMClass using xprop, I tried, but without success.

I'm not a c developer, but is it the place where mate configure the icon?
line 635

It seems the icon come from a resource inside the executable,
function: panel_key_file_get_locale_string