https://wccftech.com/galaxy-s9-pie-build-reportedly-causes-massive-battery-drain-for-some-users/

Although it is technically a ‘stable’ release, Samsung’s OneUI-based Android Pie isn’t completely bug-free. So far, nothing major has surfaced just except for one that causes the battery to drain rapidly. The early rollout could very well be a test and the actual one might begin as scheduled in January. Samsung has had a particularly estranged relationship with batteries ever since the Note 7, and it looks like that spectre is back to haunt them again. Even some of their Gear smartwatches had the same problem for months, but it appears that it has been fixed.

First spotted by some users on Sammobile, the bug caused some Galaxy S9+ users’ battery to drop from 10% to 5% in a matter of seconds. Initially, it was thought to be due to background app activity, but that doesn’t seem to be the reason. Considering that only a handful of users have received the update, it may be a device-specific issue. If enough users report it, a fix might be baked into the global build.