When a person reviews a PR and subsequent commits are made to the PR, the Reviewer section is updated to show that the review applies to an earlier commit e.g.

Reviewers

    Joe Smith (@jsmith - Reviewer) ⚠️ Review applies to 77edb59

The commit hash is a link to the commit, but it is a link directly to that commit alone - as [~kcr] put it:

"it lists the HEAD of the PR branch at that time."

but what they actually reviewed was all commits up to and including the named commit.

By clicking the link you get an incorrect view of what was actually reviewed and may think that significant changes have not been approved by a reviewer.