Automatic hibernation in non-root mode cannot work with secure key-guard.

Sorry, it's not an intentional design decision but a technical limitation. Since Greenify needs to unlock the screen first to perform the automated hibernation, secure screen lock stopped that for security reason.

It's suggested to instead use the convenient "Pull-up Gesture" on Home soft button to hibernate apps and turn off the screen altogether. If your device has no bottom navigation bar with Home soft key, I'm afraid the only viable solution is to use the "Hibernate (and Lock Screen)" shortcut (aka widget), probably with some gesture launcher tool (e.g. SwipePad).

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