Lost my exp, hence lose the streak :(
I have faced this problem several times. I completed about 2 lessons (my goal is 20exp/day), the notification showed that I was on my streak. But just few minutes later, when I checked my profile, some exp have gone, and I had to learn another lesson to maintain the streak... I don't mind studying more than I should, but it's pretty annoying since I've lost my 21 day streak thanks to this kind of issue... :(
P/s: I mostly use Duolingo on my iphone.
If you are using the new app and reviewing lesson (or doing Strengthen Skill exercises) you may not actually be earning 10XP per lesson.
If the lesson you are reviewing is already strong, you may be earning as little as 3XP each review.