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This is by design. If you unlock the phone and know a PIN for one app, you know a PIN for all of them, don’t you?
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This is by design as well. Why would you be asked for a PIN after short amount of time if you entered it before? Why would you hand your phone to other people after unlocking an app?
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This is a limitation of the OS, not much we can do about that (I could look at it to see if there is any workaround, but I doubt I’ll have time for that).