YouTube widget is shrunk with the latest MNML theme


I’m starting a personal blog, nothing fancy. It’s running the latest MNML theme (right from the master) and Ghost 4.4.0. reported that theme is 100% compatible with ghost v4 but I have a problem - YouTube widget is exceptionally small Connecting to Lightning Network with Elixir — Yevhenii Kurtov's blog

If I manually set height and width properties on the iframe element in the dev tools then it’s all fine.

Is this a problem with a theme or something else? How do I fix it?