Just started using an Android phone and left it overnight to see how long it would last. After about 10 hours the battery is still at 100%, so could there be some kind of delay when it comes to the battery percentage or is it something else? My phone is a Leagoo Lead 4.
You could try some apps like Battery Monitor to check the usage of all apps. But your let's say "problem" is not so unusual. If you want to be 100% sure about that, try it this way. Completely drain the battery out, sou your phone turns off. Then plug it in to the charger for 8 hours. (That's enough to charge it to 100%). After it has been charged plug it off and leave it for 10 hours. But! Don't install any new apps on it. Just to be clear, apps like Viber , Whatsapp,Messenger,Facebook... even if you closed them they are still running, because if not you would not get messages. Besides display and games these apps drain most of your memory (RAM) and battery. I hope this helps.