Third party App locker (such as mine AppLock AppLock | F-Droid - Free and Open Source Android App Repository) 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 ( Restricciones para iniciar actividades en segundo plano | Desarrolladores de Android | Android Developers).
Note:Stock Android doesn’t a privacy App lock as most of stock ROM have inbuilt app lock