Review one lesson, strengthen a different one
So I've just completed a review of Prepositions 3, in French. At the end I got the fanfarre and guess what - you strengthened Flirting! And Prepositions 3 is still up for review, like I hadn't touched it at all.
Another bug from a long string... I'm giving up reporting, no one seems to be looking at it anyway.
That's not really a bug. Duo's strength estimates are based on the individual words in the skill. Since sentences typically draw on vocabulary from multiple skills it often happens that you go up a level in skill other than the one you were practicing, particularly if you were near the threshold already. It also happens regularly that you improve duo's opinion of how much you know from a skill without actually going up a level. You just happen to have run into both phenomena at once.
That would actually make sense, but I don't think it's what happened. I had Prepositions 3 nearly full, while Flirting was near the bottom. If anything, it would make more sense that Prepositions 3 had been topped up, as it only needed a little nudge anyway. Not to mention that there was only one odd sentence from the Flirting lesson in this review; it's easy to tell, because that lesson is all about full expressions, not at all about individual words. Can't see how I would have strengthened Flirting from practicing a bunch of totally non-flirty words!
That's what I meant about being near the threshold. You were just below the threshold between levels in flirting, so close enough that a single sentence pushed you to just above it. Say, going from 1.95 leaves (or whatever duo calls them) to 2.05, but duo of course only shows an integer number of them. In Prepositions 3 it was presumably the opposite. Your strength increased by more, but didn't cross the threshold.