Why does my phone keep shutting off at around 50 - 60%?

Hi, about two and a half months ago I decided that it would be time to replace my battery within my S6. I purchased all of the required parts from ifixit and was successful when it came to the replacement process. After following the recommended steps in battery calibration, I noticed that the phones battery life never really got better from prior to the replacement. Fast forward to about 3 weeks ago and the phone now only goes from a full charge to only about 50 - 60% then completely dies. When I go to charge the phone, the phone acts like the other 50% is still there but it just isn’t being used. I’ve already tried factory resetting the phone but that led to nothing. If anyone knows how to help I’d be much appreciated.