Oxygen OS 10.3.0 addressed multiple issues for the OnePlus 6 and 6T, but it also broke the notch hiding feature that had been present in Android 9.0 Pie builds. While OnePlus is yet to fix the problem, there is a workaround that may do the trick.
https://www.notebookcheck.net/Oxygen-OS-10-3-0-breaks-notch-hiding-feature-for-the-OnePlus-6-and-6T-but-there-is-a-workaround.449390.0.html