You said that you bought the phone 18 months ago. The Battery Health normally declines about 1% a month on average, so your actual Battery Health is in the 80-84% range.
Another way to say the same thing.....the battery in your phone was designed to go about 2 years before needing replacement.
That 100% that you see is make believe. The shop that reworked the phone messed with the battery settings. No way that an 18 month old phone has a battery with 100% Battery Health.
This like buying a used car that runs on gasoline, you drive it 300 miles and the gas tank still reads full.
Make an appointment at the Apple Store / Genius Bar and have them check the battery. You will find that it is probably past the point where it should have been replaced.
That is why your phone is shutting down. A new genuine battery will fix that issue. About $95 (U.S.)