I suspected that too, but I do not know enough about these reactive frameworks to be 100% sure. But as a user of such apps, it is absolutely frustrating.
To stop jumping you need to explicitly size yet-to-load sections. Without JS this means giving images explicit sizing, with JS this means any section can be dynamically loaded and, therefore, jump. So good design takes into account dynamic loading and places size bounds accordingly.
Reactive libraries/frameworks don't explicitly make this worse or better, except their presence implies a high chance of dynamic loading and, therefore, more opportunity for bad design. In addition most component libraries fail to communicate /who/ needs to size a component and if it is ever dynamic. It really doesn't help that most 'official' examples fail to resolve these issues.