Battery keeps draining after shutoff

I recently got a second-hand iPhone X handed down to me, so I'm not overly familiar with all its quirks.

My phone continues to drain the battery after I shut it off. If I turn the phone off overnight without plugging it in. When I try to turn it on the next morning the battery is completely drained and I have to plug it it and wait for it to charge up enough to turn back on.

Is this normal behavior? Is there a setting to change this? I did some searching online but didn't find a solution.