NoLowPowerAutoLock V0.02 is released(developer Spark ) which helps you prevent 30 second screen lock and dim (optional) when your device in low power mode. You could add his source from Cydia: https://repo.xarold.com/

Some users are complaining about this problem, and not seen a decent solution. But now when you enabled this tweak will prevent the screen locking when in Low Power mode. You can also set it so the screen still dims, but the device does not lock. You could

Package name: NoLowPowerAutoLock