> if the manager is thinking about and getting involved in architecture after the PR is written it does suggest something has gone wrong. If there are architectural considerations then it is good to discuss them with the coder before they start developing.

That's the ideal yes. The problem with poor design/architecture is that it's never actually architected and designed. It just happens as part of a process where someone codes something without actually considering this to be a "design" (something that will affect future code, and solidify over time).

So the job of whoever it is (senior developer, manager, colleague, ...) is to point out the poor design. The hope then is that it can be fixed before it is merged AND that next time there will be no "accidental design".