I suspect that your update manager/software updater settings have 'update to latest Ubuntu release' has not been altered to 'Never' - you could have a broken install if this is the case. If update to latest ubuntu is set to 'Never' then right-cllick the 'ubuntu' logo of the menu and select 'Preferences' - then go to 'Preferences' tab and check mark - use distributor logo to get the 'Z' back.
So, I checked and the Use distributor logo instead of button theme was Checked (with a check mark, just to be clear). I unchecked it, rebooted the menu, the icon "crashed", I clicked on it to restart... and the Z appeared.
I went back in to the preferences and checked the Use distributor logo.... I rebooted the menu, the icon "crashed" again, I clicked on it to restart... and the Ubuntu icon appeared.
So, I once again went back to the preferences and unchecked the Use distributor logo... etc. and the Z is now back.
thank-you