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Turn off stamina mode,go to settings-apps-all-eco mode controller,force stop,clear data and force shutdown the phone.
You are welcome.Set a correct answer for others to see.
Clearing eco data is not a fix and your phone is not broken, stamina mode estimates change depending on your usage of the phone. Clearing data resets this, it will go back to saying 12 hours in a couple of days if your usage is same as before.
This estimate has no impact on real world battery performance, its best to just ignore it.
I never said it is a fix,it is just a way of resetting stamina mode,for his sake.From now on,he can modify the battery life if he wants.