Hi,
just tested it on 4.3 (which should be pretty much the same as 4.2 regarding the App Locking) and all works fine for me.
- Installed AMS from Google Play.
- Went to App Locking and locked random app, AMS asked for a new PIN which I entered.
- Went out of AMS and tried to run the random app, PIN dialog appeared.
- Pulled down the notification bar, selected AMS which appeared instead of the PIN dialog.
- Pressed back, the PIN dialog appeared again instead of the random app.
What phone exactly do you have?
Filip