Duolingo tracks your progress on the next day?
Whenever I get XP on Duolingo, it always fills up tomorrow's XP goal. For example, today because I worked yesterday, it said my goal had been reached for today (Friday) despite not doing any exercises, yet when I did my exercise it filled up Saturday's XP goal. This is annoying because sometimes you work on a day but it doesn't work because it fills up tomorrow's XP goal rather than today's, meaning I lose my streak.
Have you moved time zones since you first registered with Duolingo?
Duo uses the time zone you were in when you registered and it cannot be changed. If you have moved then you may see this effect if you don't adjust your time to the original time zone.