My typical workflow - in private business repos, but for the same reasons specified in OP - is as follows:
1. Fork the PR branch.
2. Make the changes I want to see upstream.
3. Create a PR into the author's branch.
4. Ask the author to review my changes.
5. When my PR is complete, finally review and merge The original PR.
This gives me the opportunity to explain my changes in commit messages, and gives the author an opportunity to push back or argue actual functionality changes I made if it's not all coding style. Now the author has license in accepting my changes. It avoids the cumbersome comment/resolve feedback loop, shows the author that their contribution is important and I'm willing to put time into making it right, and ultimately gives them credit when their PR is merged.