According to SamMobile, the Android 9 Pie update for the Samsung Galaxy S9/S9+ and the Galaxy S8/S8+ brought some unintended issues to these models. Many users have complained that the files containing the latest build of Android and the One UI interface contained a bug that drains the battery on their phone, lowering battery life.
Owners of affected units say that they have seen their screen time drop by as much as 50% because the battery inside their phones run out of fuel quicker. The manufacturer has not yet commented on the complaints, and while it isn't totally unusual for the installation of a new Android build to cause a change in battery life, the extent to which these batteries are draining is obviously excessive.
Some users are suggesting that those with a 2018 or 2017 Samsung flagship negatively impacted by the update, try a factory reset to see if it helps with this issue. Another user recommends letting the battery on an affected phone recalibrate. To do this, let the battery drop down to 0% and charge it back up to 100%. Repeat this process, and the battery should improve over time.