Battery

Battery percentages on cell phones are not an exact measurement. The phone measures electrical pressure and this pressure is handed off to an algorithm to yield an estimation of battery percentage remaining. This is why the percentage will often change after restarting the phone.

What is your battery health ?

go to Settings > battery > battery health.

Not sure how to help with your issue specifically but you may find some help here: iPhone Battery and Performance - Apple Support

My iPhone 6s Plus battery indicator did the same thing. It would show it was 50% charged then suddenly drop very low & at times would shut off saying I needed to charge it. Then when I plugged it in, it would show it was 50% charged. Very erratic. My battery health was down to 80% on that phone so I'm not sure whether that was the cause.