This seems to be a random failure, sometimes all videos work, sometimes a video or some of them cannot be played. It happened in a page containing embedded videos. This already happened in the v3.9.5 app.
In my tests, if I left the page and opened it again then the videos always worked.
Using the inspector, if I duplicate the <video> HTML tag that isn't working then the new one works fine, so maybe it's a WebView bug or a race condition.