Commands API: Some Commands Broken?

[Patrick Dark]

I can’t seem to get some command combinations to work at all, such as 
those involving “Comma” and “Period”.

Test code used in Firefox 53.0a2:

"commands": {
     "Duplicate Tab A": {
         "suggested_key": {
             "default": "Ctrl+Shift+Y"
         },
         "description": "Duplicate the active tab."
     },
     "Duplicate Tab B": {
         "suggested_key": {
             "default": "Ctrl+Shift+Comma"
         },
         "description": "Duplicate the active tab."
     },
     "Duplicate Tab C": {
         "suggested_key": {
             "default": "Ctrl+Shift+Period"
         },
         "description": "Duplicate the active tab."
     }
}

let test = (command) => {
     console.log(command + " was activated.");
};
browser.commands.onCommand.addListener(test);

In the preceding test code, only the Ctrl+Shift+Y command does anything.