Screen lock and lock screen timeouts are conflated

1. Set Security & Privacy -> Lock phone to "After 10 minutes"
2. Lock phone (or wait 10 minutes for it to lock).
3. Receive a text.

Expected behaviour: screen turns off very soon after to conserve battery (I expect between 5 and 60 seconds).

Actual behaviour: screen remains on for 10 minutes, draining battery.

Even worse, if I set the lock to "Never" then after receiving a text the screen will never turn off. In this case I expect to take responsibility for turning the screen off when I am done using it, but I expect the phone not to turn the screen back on when I am not around and receive a notification unless the phone does time it out in that case.

I think the screen lock timeout should be completely separate from the lock screen timeout. I would like to set 10 minutes or never for the screen lock timeout, but I still want the lock screen to time out and turn the display off after a few seconds.

I found bug 1420493 which is in a related area but doesn't refer to the lock screen timeout itself. My failure case is a use case about battery drain, rather than needing to repeatedly re-enter the passcode.