How do I get a lone missing app to appear in Spotlight searches?
While this does not directly answer as to why that particular app does not appear in Spotlight, nonetheless as a workaround, try the following:

Explicitly exclude the App, via drag and drop from Finder, into System Preferences > Spotlight > Privacy, and then close/reopen Spotlight preferences and remove the App by selecting it, then click the minus button [-] at the bottom left of the list window.

The App should now appear in Spotlight, however it may disappear again if the root cause is triggered again.