I feel like this might be some WordPress derpy-ness. It looks like the resource is loaded and displayed before its style is applied. It might be the case that if this was done mobile-first (i.e. the centering was the default) it would work just fine. But I’m not sure I really know what is happening to be honest and debugging WP code in the browser is kind of difficult.
If nothing else can fix it, using a loading spinner might at least hide the page until it is fully loaded.