iPhone 6 shuts down when battery charge reaching 50 to 60%

My iPhone 6 shuts down when battery charge reaching 50 to 60%, Back up done, Restore Done, but still it does not work as expected, Went to Apple Service in my location near chennai, Battery looks fine as per their diagnostics.

In case issue in battery they told, they can replace battery, in case some thing else, they told, i will need to replace and buy new phone, they will take back my old phone. Is this expected process from Apple? How do we know what the issue is and why the issue came in just one year.