iPhone Battery drains fast while doing Basic Tasks

Hi,

my iPhone 6s has the following Problem:

When I am e.g. watching YouTube Videos, the Battery lasts for a couple of hours, just as expected. But when I am just swiping around on the Home Screen, it looses its Capacity very fast (around 1% every 15 Seconds).

After rebooting/resetting the iPhone, the Capacity was higher again.

The Battery health is at 85%, according to the iPhone, but on iMazing (PC) it always jumps between 50% and 80% for some reason.

I already tried reinstalling iOS and to calibrate the Battery: Both things didnt work.

The Battery is an Original.

What should I do now? Should I replace the Battery? Or maybe it isnt even the Batteries fault. Can it be the Logic Board as well?

I hope you can help me.