Strange Battery Issue Capacity

I’ve changed an iPhone 6 battery twice now and it will show 100 percent capacity in settings battery and also coconut battery and 3utools. I also have a battery tester machine and shows the same thing. Anyways, after about two weeks, the battery is at 70 percent and the settings says service. I’ve tested with 3utools and my machine and also shows 70 percent… This happened twice now and I’ve never had this kind of issue before. Would a u2 ic chip cause this issue? Or is this battery related? The phone has been wiped dfu mode etc and nothing has changed