Chosen Solution

I replaced the battery of my iPhone 6S. It had 75% max cap. and my question is if it is possible to get under 100% after a battery replacement cause I don’t want to replace it again.

Max capacity will always deplete over time. The aftermarket batteries will drain and deplete faster so make sure to get the battery replaced from a reputable store or buy a good quality battery.

check this post [iOS 11.3] Battery Health Beta - No Change after Replacing Battery