This needs to be fixed by the Mint devs . The problem lies in the Mint-X GTK theme, but can be fixed quite easily. Open the following file:
sudo gedit /usr/share/themes/Mint-X/gtk-2.0/Styles/menu.rc
Find the following line (around line 14):
fg[ACTIVE] = @selected_fg_color
and replace it with the following:
fg[ACTIVE] = @menu_fg_color
Save the file. The hidden menu items should now display correctly. Note that you may need to manually trigger a reload of the theme. The easiest way to to this is to log off and log back on again.
 I've submitted the fix to the bug tracker. The answer here on SU serves to help the people affected by this problem until the patch makes its way into Mint.