Q: Low Power Mode bug?

Hello all,

My iPhone 6s (iOS 9.3.1.) is repeating the same behavior at every second-third time it's charged. When the phone reaches 20% and pops up the dialog to enable "Low Power Mode" or "Close", if I choose "Close" the phone shutdown. If I try turn it on again, it shows me the sign to connect the charger. If I wait a few minutes, it turns on and show me 20% battery, but as soon the Low Power message pops up the process restarts all over again.

Again, this bug doesn't happen all the time. It seams to be related if I'm using WiFi at the moment the phone reaches 20% of the battery and displays the "Low Power" message. Also, the bug is not related to when the phone reaches 20% of the battery, but when the Low Power Mode message is displayed. So it can happen at 19%, 18%, 17%, etc.

Any suggestions or ideas how can I get rid of this annoyance?

PS: My old iPhone 5 suffered from the same issue since I update it to iOS 8.