I presume these announcements are written by the engaged parties. In which case, it may be that they are trying to reassure people who recognize their names, but haven't spoken to them for years, that they are not committing bigamy.
Otherwise, yes, it sounds like a very strange custom. Sort of like putting in a graduation announcement, "We are pleased to announce that Stephen Smith has graduated with a B.A. in English. He originally planned to be a doctor."