For certain tracks using lanes, some items do not display the utility buttons (Mute, Grouped, Locked, etc.)
The following licecap is from a fresh portable install (4.591, Win 8 64bit) + license key + ASIO, and the setting to show the "Unmute" button over the media items is enabled.
Note that the mute button does not appear on the left item (this is true for any other buttons that may be in play as well, but not demonstrated in this licecap) but it is nonetheless there and active.
Perhaps it is related to the presence of empty lanes (see licecap)?
I could not easily reproduce the bug from a new project: recording in a loop for a couple takes, manually adding an empty lane above the current take, etc. But after closing and open such a test project a couple times, the bug appeared. Not sure exactly how that happened, but I provide here a link to the demo project (not the one in the licecap, though I can provide that to, but the same thing...)
Let me know if I can provide more information, or if this bug is already known.
Project file is here.