Why does my iPhone 6s battery drain jump recently?

Hello, For the past couple of weeks I have had a battery drain problem on my iPhone 6s, I have contacted Apple support and they are currently working with me to find a solution. However I wish to gather more information from Apple support communities, so the problem is that occasionally when I turn the iPhone on in a morning the battery level drops from 100% to 99% then 98% then 97% then levels out there for a while, however last week I was just doing some light internet use for 2 minutes and the battery dropped 5%, I know for sure that the hardware of the battery is good through multiple tests, also it isn’t one of the affected iPhone 6s batteries that was affected by the shutdown bug. If anybody has any information regarding this issue, it would be much appreciated if you could reply back to me.

P.S. - It is a 14 month old iPhone 6s 128gb, currently running iOS 11.1.2

Thank You in advance