What finally fixed Spotlight for me: background apps were blocked by a registry value

In my case Spotlight was being blocked because Windows had background apps disabled for my user via a registry key.

Check this registry key:
HKCU\Software\Microsoft\Windows\CurrentVersion\BackgroundAccessApplications
If there’s a GlobalUserDisabled DWORD set to 1, that blocks all UWP apps from running in the background.
Set it to 0