New iPhone 6 Battery replaced by Apple still depletes fast

I got the battery replaced on an iPhone 6 last week under the $29 (Singapore $38) replacement program at the Singapore Apple store. After the replacement, I noticed that the new battery was depleting at about 6% per hour while on standby mode (sitting on the table overnight in airplane mode). The phone was put on low power mode too and background app refresh turned off. The phone was running the latest iOS 12.1. It was just reset 2 days before the battery replacement and had only 5-6 additional apps installed.

The battery health indicator was showing 100%. The health indicator on the original battery was 59%. Also note that before the battery replacement they ran a diagnostic on the phone and only found problem with the battery, which they were about to replace.

I took the phone back to the Apple store and they did a reset again and asked me to observe it. I did that for the next 3 days with no additional apps installed. However, I got the same result - a rapidly depleting battery even on standby mode. This was also corroborated by the usage graphs in the battery settings.

As a benchmark, my new battery gives me about 15-16 hours of standby time. Apple claimed up to 10 days of standby when iPhone 6 was launched. I'm not expecting 10 days of standby. But I do expect about 36 hours (1.5 days) of standby from a brand new battery under the following conditions:

1. Latest iOS (12.1) installed by Apple store

2. No third party apps installed

3. Background app refresh turned off

4. No apps running in the background

5. Phone in idle state (no screen usage, no phone calls).

6. Very strong cellular network

I tried other things as well such as enabling low-power mode, or putting the phone in standby mode and wifi turned off, but the battery still depletes quickly.

I took it back to the Apple store yet again. They tried to convince me that since the battery health indicator is showing 100% and it showed green in the diagnostic test, and that they had already reinstalled the latest iOS, it has to be a problem with the logic board.

I argued that if it was a logic board issue it should have shown up in their diagnostics too since they were telling me that the battery is good based on the same diagnostic test. How can the diagnostics be trusted for the battery, but not for the logic board?

Moreover if there was a problem with the logic board or there was any chance that battery life would not improve, they should have warned me in advance, but they didn't give me the slightest of hint.

I tried to talk to the store manager but he refused to help too. He asked me to escalate through the Apple support hotline. I did that but they said they will talk to the store and get back to me. The case is still open.

I have reason to believe that the replacement battery is faulty. I saw other threads where people have reported similar problems. I want Apple to diagnose the exact problem with the phone or if they can't do that, refund my amount back to me.