Third party App locker (such as mine AppLock https://f-droid.org/app/io.github.subhamtyagi.privacyapplock) may not be work in Android Q.
As ‘App locker’ apps run a service in background(Foreground service in Oreo) and starts a foreground activity (i.e. lock screen) whenever a locked apps opened. but as proposed in Android Q background service can’t start a foreground services ( https://developer.android.com/preview/privacy/background-activity-starts).
Note:Stock Android doesn’t a privacy App lock as most of stock ROM have inbuilt app lock