iPhone 6S battery problem

Hi,

for last 3--4 months I have problems with my iPhone 6S 64GB battery.

Phone sometimes switch off while battery is around 20-30% and shows like battery is empty.

When I connect phone to charger after few minutes phone turns on and shows battery level around 20-30%.

This happened 15 times in period of 3-4 months.

In this period I did clean install of ios 9.2.1 , 9.3.1 and this didnt fix my problem.

I took my phone to my carrier who sold me the phone and as phone is still under warranty they sent it to unofficial service for repairing.

They simply installed again new iOS 9.3.1 and "calibrate battery" and wrote in service report that everything is OK, and after 4 days again the same error, phone switched off when battery level was 21%.

I measured my battery capacity with Coconut Battery app.

Is it normal, that battery with 209 loadcycles have capacity of 85,4% (1442mAh) from maximum 1690 mAh??

Any advice?