When you get or send a dance invite, it goes into a queue. When you go to accept a dance, it just gives you the next one in the queue. This queue never forgets a single invite - it will even remember invites from last year that weren't responded to yet. So it's possible that you or your partner - or both - are still working through this queue. Remember that you may get the acceptance hours or even over a day after you send the invite, because the other person might still be working through their queue (or just not online yet)

So far we have not had any confirmed cases of dances either vanishing or being duplicated.

If you'd like, you can file a ticket to ask a UA to double-check your logs. All dance invites and acceptances are logged, so if you wish, file a ticket and include the usernames and approximate date/times of the dances you think have gone missing.