That looks like an issue we’ve seen before on WordPress sites built with Elementor, specifically when the Elementor right sidebar has the ‘stick-to-top’ feature enabled.
The only solution I found on our end (though I think Elementor support could and should help find a solution on their end) is to set a minimum height value for our iframe. This ensures it loads at its maximum height from the start, before displaying the dropdown with availability options.
This is gonna 'hide' the twitching effect, which is due to the way Elementor loads the script inside that container.