My bluetooth tray icon will intermittently disappear and reappear, usually after returning from suspend. At the login screen I can see the icon fine; when I log in though, it will (seemingly at random) disappear. However, it's still "there" in that I can click on it and access bluetooth settings and whatnot.
This happens to me when i use Ayatana (tray icons) and use more than two extra icons. For example, i use Megasync and system-monitor icons. If i add another (telegram icon for example), bluetooth icon disappears. Are you using ayatana indicators?