Does the iPod Touch's battery last longer on iOS 12.1.1 than iOS 11?

My sister has just got a new iPod Touch 6G for Christmas running iOS 11.4.1. The battery is acting very wierd:

It never charges past 99%

It goes from 99% to 20% in around an hour

It stays between 20% and 15% for 1-2 hours

It goes from 15% to 0% in one minute then shuts off

After it powers on it has 17% (most iPhones/iPads turn on at 3-4%)

Does iOS 12.1(.1) fix any of these problems?

Or does the battery life decrease?

Cillian