Low battery mode remains active when after phone is charged

Hello,

my wife's iPhone (6s Plus) goes into Low Battery Mode once below 20% which is all nice and good. However even after a full recharge it remains in Low Battery Mode and stays that way unless we manually set it off that mode. This happens every time, and has been happening for a while, regardless of iOS version etc (we are on latest always anyways).

I see a few old post talking about the same thing but nobody seems to have a solution.. just testing the waters now.

Thanks