It's usually because you are using another activity as your app's main activity. UniWebView's activity must be the main activity of your game, to prevent some Unity issues which might cause your game loosing focus. It supplies some workaround on background/foreground switching while the web view is being opened.
To solve it, you need to update your AndroidManifest.xml file to use UniWebView's activity. Please refer to the Merge Guide and/or Recompile Guide section of our manual and follow the steps there.