iPhone 6 Battery replacement only lasts 4 months

I still use my iPhone 6 and had the battery replaced as part of the iPhone Battery Replacement Program In Jan 2018. I made an appointment for almost 2 weeks later (couldn’t get in sooner) and went to the Apple store only to find out they didn’t have the battery in stock... But then had to wait another 10 days for the battery to be available. Then I went back to the Apple store, paid the $29 and my battery was replaced.

After which, my phone worked great!! Fresh battery & latest iOS it was like a new iPhone. The battery section under device settings said I was running at peak performance.

However, I started having battery issues again in May 2018. I began to see excessive draining and all the normal characteristics of a failing battery. I looked at the battery section under the device settings and this time it said I had 100% capacity, but told me I needed to service my battery.

I made another appointment for a few days out (much quicker this time). The Apple Genius tested the battery and was surprised that it was failing so soon after replacing and said he thinks the battery replacement I had gotten in January was faulty. But unfortunately I had to paid the $29 AGAIN and after about 2 hours, my battery was replaced AGAIN.

WELL... Here I am AGAIN in September 2018 and my phone just started to die AGAIN with about 35% battery available.

Should this be happening??