I doubt it's prompting. I think the issue is more likely that UI code is often similar, has a lot of examples online, and often doesn't require understanding data flow. This is why LLMs are great at "boring" React components, because they don't actually understand the flow of the data, but they don't need to.

Business logic on the other hand is much more likely to be novel in some way, there are likely fewer similar examples for rules to be learnt from, etc.

Obviously this is all gradations, LLMs can manage some business logic and mess up some UIs (they can't "see" the UI which doesn't help!), but this is my experience of them and fits with my understanding of the technology.