This has only been happening the last few weeks. It doesn’t happen often so I can’t be certain it’s related to the new interface. It’s happened on several languages and collections, but I’ll try to make a note next time I see it.
The bug is like this: After doing sentence N, you hear sentence N+1, but the cloze that comes up isn’t that of sentence N+1, but that of sentence N, repeated. You do it and then you get sentence N+2 – sentence N+1’s cloze has vanished.