Sounds like the problem is a unit of work is split between too many people, i.e. how the people in the example are using PRs, not PR (as a tool) itself.
My experience is that each PR belongs to one person, but multiple people often have the necessary knowledge to complete the work. Especially when a "senior" is working with a "junior" to help them complete the work. On a big codebase, it can often just be knowing-where-to-look.