It seems like 'some' internal functions are not allowing icons to be shown when added in a context menu.
For instance: FileType CONTEXTMENU {7A0B5906-6671-42FE-B404-6247FC57770B} CONTEXTFORCE does not show the assigned icon in the menu, whereas: Clipboard COPYNAMES=unc does. Sub-menu context items also DO show the icons ok, so far the only command I see not showing the icons is the FileType CONTEXTMENU command.
The thing is, the Filetype CONTEXTMENU command is a dynamic command. You never actually see that command entry itself - it is replaced by something else, in this case, any context menu entries added by the specified handler. So what you will see are the icons (if any) added by that handler.