Things like WhatsApp, tasker and 3c's battery monitor app run continuously in the background. As they're exclusions in stamina mode it effectively disables stamina mode.
It's meant more for apps that are only active while running, like BT connections or uTorrent or something like that if you get me.
So eg, I add an exclusion for the the sbh52 app which is only activated when the unit is paired, during that time stamina mode adheres to the exclusion and prevents the phone from sleeping, then when I disconnect, the sbh52 app autocloses and stamina mode reactivates sleep.
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Sony Xperia Z Ultra C6833
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Samsung Galaxy Note GT-N7000, iPhone 3GS, HTC Touch Diamond, HTC Touch Pro, HTC Universal, Sony Ericsson P990i, boat load of nokia's for a long time.
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