CSV upload says "existing registrant ignored" - but not in list

A webinar shows 21 registrants. I upload a 22nd from a CSV, but receive an "existing registrant ignored" message. Yet when I check all 3 pages of registrants, they do not appear anywhere on the list and the count remains 21. Has anyone else experienced this? Are they actually registered even though they do not appear on the list?

When I upload a CSV of all 22 registrants to a new webinar, all 22 appear. So it is not that this user is banned by Zoom or the like.