In any video preview while you're in the Twitter app, the duration part and the video part are closed with a white layer. The same happens in the articles in the profile section of the application. This happened after you installed iOS 14 beta 1. Should I go back to my old pack?
It looks like you found a bug. It would be super helpful if you could submit a bug report with the Feedback Assistant app on that device running iOS 14, or use the Feedback Assistant web page. That will get the info to the right teams to find a workaround for you and hopefully make a fix in a future release.