Bug in Streak display.
So late last night (about 23:50 CST, 2017-07-13), i was getting in some practice in Japanese on the android app (latest version as of the time of this post). When i finished, and Duo said "You've met your daily goal!" i just hit the home button on my Pixel, dropped my phone, and promptly passed out.
This morning, I got into work, hit a low spot, and pulled my phone out for some practice. I unlocked the device, hit the "switch app" button, chose DuoLingo, and it said "You've met your daily goal!", played the little trumpet noise, and showed that my streak was continued for today (2017-07-14), despite me not having done any practice yet.
When I hit continue, the streak indicator at the top of the screen showed no streak again. It appears to just be a display bug.
No biggie really. It has happened to me a couple of times, but can be easily fixed by getting the required XP needed for that day, even if it doesn't revert to its old self and show you haven't completed yet.