(name) asked to join (Facebook Public Group name)

I am a mere member of this group and am not, and never have been, an admin. (Not that there's anything wrong with that.)

The link underlying the "Respond to Request" button brought me to the genuine "Member requests" page with this one request as its lone item.

I've never seen this failure before. It's not a meaningful "breach" in itself, but if it indicates a broader defect in Facebook's auth process, yikes.