I noticed that the Tor Browser icon does not appear when it’s open in the panel’s “Task Manager.” Instead, it shows a generic gear icon. 
This is resolved by adding the line StartupWMClass=Tor Browser in the file /usr/share/applications/janondisttorbrowser.desktop with root access.
Then, restart the lxqt-panel to see the change.
pkill lxqt-panel
ALT+F2 lxqt-panel
By the way, is it possible to see the WM_Class somehow in Whonix 18?