every week until now I've had tons of successful defenses, hell last week I had some not even an hour after the reset. But now I even have a better red unit (Lucina vs my -atk Lyn) and nothing yet. Don't really have much I could switch out that I think would perform better, I assumed my Robin/Olivia combination was getting me defenses before.
Strangely enough I usually have my 10 defensive wins by now myself. But I only have 1 as of now. I think it's because people don't have the arena units leveled up so they care less about getting a perfect run. That's my theory anyway. I don't think the meta changed that much but maybe it did.