It's probably OOS 9.0.4 on OnePlus 6 doesn't use webview. You can see page 155-156 or bromite's wiki. There's a clue there.
You can enable webview once you disable chrome on OOS. I'm on 9.0.12 on the 6T, but regardless of enabling it, I havent been able to replace it with Bromite's version. Going through the normal process on another phone doesn't seem to apply to OOS.