My mom was complaining that her phone battery was lower than it should be, and when I checked the graph I saw this. It looks like the battery suddenly jumped from just over 50% to around 25% around midnight for no apparent reason, and stayed exactly there for many hours. She didn't use her phone around that time at all. Is this a sign that the battery is failing, or just a bug in iOS? Anyone else seen this before? It's a slightly over four year old iPhone 6.