Your battery is knackered. Time to get a new one.
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It‘s very difficult to prove a negative, really the onus is on you.
Most people here are just posting their experiences, they want to continue using their devices as before, and not have them actually be worse after performing an "update". I don't know if you have apple stock or something, but I can assure you that this is happening to my own iPhone SE, a colleagues iPhone 6s (he showed me himself), and to many people across the forums.