A few OnePlus 6 handsets are reportedly suffering from screen flickering issues when the adaptive brightness feature is enabled. The handset seems to flicker between two brightness levels when the brightness goes high. OnePlus is yet to acknowledge and provide a fix for the issue.
https://www.notebookcheck.net/OnePlus-6-s-adaptive-brightness-feature-reported-to-cause-screen-flickering.319078.0.html