This may already have been reported. If so, I apologize for the duplication. I have noticed that in both French and German pressing the Explain Button does not always work as I would expect.
Sometimes pressing the explain button results in nothing popping up at all. (or does that mean it has no explanation for the sentence?) But worse than that behavior is that after pushing explain just once, an explanation may pop up at the beginning of the next round of clozes when it was not requested.
This behavior occurs on the website – not sure about the phone app