The UI tray icon being displayed, generally happens well after the main avast service starts - try this, soon after boot open the task manager and check what processes are running for avast, you should see avastSvc.exe and avastui.exe ?

If the avastui.exe is there, but no tray icon, something is blocking that.

The avastui.exe isn’t a security based element of avast, but the interface and it also performs other interface tasks, but not security related.