I’ve rechecked the issue on my end and it seems that there is an error with your theme. The theme doesn’t have the top of the page defined, as shown below, and our editor does not know how high is the page while loading a Landing page.
With the default WordPress theme, it seems to work as expected. The workaround, in this case, is to load the default WP theme that I’ve just installed, add the Landing page, and switch back to the theme you are using. After that, the editor seems to work.
thank you so much