This issue is reproducible across all pages, even a new page without anything else on it.
I compared the page-builder.css and on the html field
<iframe class="ktc-form-control ktc preview-frame" srcdoc="froala_editor code">
The height on this field is overwritten by the builder.css, on the site that is working but it is not overwritten in the css on the broken one and height remains at 32px instead of 144 as defined in builder.css.
Im unsure as to how this is occurring or how to resolve it.