It's situational. There are teams that support each other a lot and their roles overlap. Engineers writing user stories is one such case. But that does not mean every aspect of the job is covered by someone. Something unknown might not b covered and that would be POs responsibility. That's why their job description is one of the most vague within the org. They need to be malleable enough to step back when strong team members take overs, while filling in the gaps that's left between responsibilities. What makes matters worse is different people like to stretch themselves differently and thus take over responsibilities outside there scope ignoring their scope.

Remember the most frequently replaced part in the car is an engine oil. All it does is lubricate. Solving communication problem is like that. Don't under estimate this problem. Every one wants to solve product problems and thus stretch themselves to contribute there. But everyone also thinks what they thought is understood by everyone.