Note that this issue doesn't happen with almost all Windows apps I use, so it must be some kind of bug.
I have CCleaner set to maintain a tray icon that is persistent whether or not I am engaging with the user interface.
Every now and then, I have a need to restart Windows Explorer, whether by me manually restarting it in Task Manager, or there's an app installation that automatically does the restart.
Upon restarting Windows Explorer, CCleaner's tray icon disappears and doesn't come back. Re-launching CCleaner (e.g. double-clicking a desktop shortcut) doesn't even bring it back. I have to go into the Task Manager, end CCleaner's task, then re-launch CCleaner to get the tray icon back.
CCleaner version: 5.77.8521 (64-bit)
Windows version: Windows 10 Home Version 21H1 (Beta Channel), Build 19043.867, Windows Feature Experience Pack 120.2212.3030.0.