Its one of those little annoyances where a CSS fix works within the browser but NOT when added directly to the site. Which means something else is at play and that in this case is the JS Lazy Loader.
Anyhow – the issue is because WP Rocket replaces the video embed with its own player code. But it doesn’t take into account the WP Video Block wrappers CSS. So it ends up with double the height it requires….
You can try adding the videos using an embed code instead of the Video Block:
https://embedresponsively.com
OR LAST MINUTE IDEA – try this CSS:
.rll-youtube-player {
margin-top: -56.23%;
}