I have selected an app to be hidden with Face ID, and it does not appear when i search for it. Nevertheless, when I move to the "Time of use" of the device, it is ranked among the apps that i have used, so you can realase that i currently have this app. I think this should be avoided.
When you set an app to be hidden, you see a popup that says, "App content will not appear in notification preview software Spotlight". It doesn't remove it from every aspect of the OS.
If you think it shouldn't be seen where you say, then may I suggest you raise a feedback report with Apple? It won't really get any traction just here as a post in a forum that's supposed to be for third-party developers, i.e. not Apple employees, to discuss issues in their code.
You need to raise each issue you find separately at https://feedbackassistant.apple.com/ You can post the FB numbers here if you want, so that others can link to them.