Knowing which update it is might help, since some updates cause problems and are known.
You might try doing a Search for Updates from the Windows Update panel. It may reset the database that is not showing the update installed.
If you have the option to uninstall the update, you might do that. Then hopefully it will install correctly.
Since you said you have troubleshooted, not sure exactly what that entails. But running the troubleshooter in the Control Panel for Updates as Admin might find a problem, or the link below. How do I reset Windows Update components?
You might be able to search your registry for the Update KB number. You should find an entry in the registry like the one below. On the right of the window, you might check the "CurrentState" entry and see what it shows.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Component Based Servicing\ApplicabilityEvaluationCache\
I am not suggesting you change anything because I am not sure what the entries should say, but if the ApplicabilityState entry does not agree with the CurrentState entry, I will do some research to see what it means.