The OxygenOS 5.1.6 and 5.1.8 updates to the OnePlus 6 has caused a number of users to experience weird battery behaviors, with some claiming their devices now offer half the regular screen-on time.
https://www.notebookcheck.net/New-update-affects-the-OnePlus-6-s-battery-performance-and-not-in-a-good-way.310646.0.html