> "Does this PR actually do what the ticket it is claiming to implement actually intended?"
Let me ask about your unspoken assumption: is this the PR reviewer's job? Maybe the PR seems to implement what the ticket asked for, then after merging it becomes clear that it didn't fully implement it, or the business stakeholders are unsatisfied, etc.?
It's not the PR reviewer's job to go actively test it out (you can assume that your colleagues are somewhat competent at what they're doing), but if you review with the spec or the issue open on the side that says to add a blue button and you see it's red, it's your job to ensure it's not a mistake and point it out to the author.
Let me ask about your unspoken assumption: is this the PR reviewer's job? Maybe the PR seems to implement what the ticket asked for, then after merging it becomes clear that it didn't fully implement it, or the business stakeholders are unsatisfied, etc.?