I'm probably really late to join the thread but here.
I'm getting the same issues, but for me it appears to be specifically location. I don't know if I'm just going crazy but it only really happens when I'm away from home. I can get my phone at home down to 1 percent, but other places reaching 20 is the death loop. It just suddenly dies at like 25 and then shows charge phone icon, then on the next try starts just fine, and "dies" after a couple of seconds.
Like the other person though, I did test it and I have a Samsung A9. I have no idea what is going on.
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It's probably no software because I'm on iOS 10, and not on the beta currently
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I don't think it would be that sudden though, I think it would gradually start to die at a higher percentage, not just go from dying at 0 to dying at 20 overnight.