According to Robert C. Martin's "Clean Architecture" you should try to structure your system in such a way as to separate low-level concerns from high-level domain concepts.
Following this logic, Martin proposes, that
the Web is a delivery mechanism—an IO device—and [...]
therefore should be seen as a low-level concern. As a consequence, Web apps should only present data they receive from the domain.
In principle, a PWA could be seen as its own enclosed system with appropriate architectural boundaries, so that the app wouldn't need to have a backend which contains the domain logic. In that case, implementing the Clean Architecture would be a fairly trivial act.
But imagine your domain logic already exists on, say, a server. How would you then avoid duplicating your domain logic into the PWA if UX concepts such as offline availability are key to your product? What if your domain logic is quite sensitive and mustn't be exposed to end users under any circumstance? (e.g. by implementing it inside a PWA)
Another way to phrase the question would be: How do you build PWAs with rich UX, such as offline capability, without exposing your potentially sensitive domain logic by embedding it into the frontend?