You're assuming it is not an old bug and that the branch is somehow kept around for other developers to re-open months or years later.

A PR for each commit seems overkill. Usually a PR is for a feature, but code-wise, a feature might require several atomic changes. For example: prepare configuration, move files around, add tests, add new code, delete old code, refactor. Each of those could be a separate commit, bringing you to a polished feature with test coverage.