I noticed yesterday that a few times, right before I would win the duel, it would say "desynchronized" so the win wouldn't count. It happened just like that so many times, right at the end before I won, that it seemed like the other person could be triggering the desync to cheat somehow.
Then something happened today that makes this even more obvious. It happened at the end of a match when I was clearly going to win again, but something new happened this time: I got paired with the same person in the next match. And sure enough "Desynchronized" comes up right when I'm about to beat them, just like before. So I'm pretty sure this person triggered it after they saw I would win in both matches. This was a grand duel and I was near the end of my attacks, so it really messed me up to lose the win.
I imagine if these people do it over and over and only allow their wins to go through, they can easily use this to win full series of duels by cheating and thus getting tons of elemental. After happening this many times and having it happen with the same person twice in a row, it seems highly unlikely that it's a coincidence. I'm sure some desyncs are real, but I'm getting the feeling that some people figured out how to cause them.