A credible user reports that the “Reply” button used to support PMs but no longer does. I don’t have an old version to test this. I don’t see anything relevant in the release notes. Has something changed?
I do know you can PM by clicking on the member’s name to pop up their card and then hit Message.
(Note we’ve customized the text to display “Reply All” rather than “Reply” to avoid confusion in the common use case of replying to the topic.)