This was observed with the latest version of Remote Utilities Host, as well as all previous versions. This behavior was observed under Windows 10 as well as under Windows 11.
If explorer.exe crashes and is restarted, or if it's being killed and then re-started, the system tray icon is not re-created as it should have been. This is most likely because your programmers have "forgotten" to check for the message WM_TASKBARCREATED and then to re-create the system tray icon with something like "Shell_NotifyIcon(NIM_ADD, &m_tnd);".
The same is true for "Remote Utilities Server", it does not re-create the system tray icon as it should either.
Please try adding your Host as well as RU Server installation folders to the exception list of any antivirus software you may have installed on your Host and RU Server machines as described this page. If you’ve already done that, please also try the solutions suggested in this guide.
Let us know if the above doesn’t help and the issue persists.