My best guess is that you’re in a time zone other than GMT. The individual course goals reset at midnight in whatever time zone you have in your settings. The overall Memrise streak shown on the left (with little blue squares) resets at midnight GMT.
For example, if you did Memrise in the morning on one day and then in the evening on the next day, you could have missed adding any learning events during a 24-hour period from 12am GMT to 12am GMT.
I have noticed that there can be a delay in updating the streak, so far it has eventually corrected itself. I’ve never noticed how long it took so it could be as Kaspian says, a mismatch in time zones.
I wouldn’t worry about the “best streak”. 
Mine is never the same as my actual streak and I live in England.
I’m currently on day 233 but the best streak says 226. 
I have the same issue. I had a 137 day streak going but my “best streak” says only 17 days!
And now, my streak has disappeared. I was so hoping to get to a year and now its gone. I really wanted to cry. I contacted memrise over two weeks ago, twice, and I STILL haven’t heard back.
Memerise’s customer service has gotten kind of bad. I’m really disappointed.
I’ve noticed this issue too, only my “best streak” was showing 117 days when I hadn’t been using it for that long. And what exactly counts as a “learning event”?
@icytwilight & @Patrick_Dunn25 - Remember: The streak for an individual course where you’ve set a goal is based on the time zone you set in your settings, but the “Best streak” shown in the left sidebar is based on GMT.
The individual course goals are point-based; you have to earn enough points to meet your daily goal (1500, 6000, or 20000). The Best streak is based on having at least one learning event each day (12am GMT to 12am GMT).
You can lose the overall Memrise streak while keeping your course goal streak if you do Memrise early in the morning on one day, and then wait until late at night the next day.
Learning Events: Each time you answer a question in a learning/planting or reviewing/watering session, that’s a learning event. If you preview a lesson, each word you view counts as a learning event. If you *overwater, each answer counts as a learning event. I believe that choosing and creating mems also count as learning events.
*Overwatering: The original Memrise term for reviewing items that are not due for review.
@Patrick_Dunn25 Based on the 120-day badge on your profile, it sure looks like you’ve been using Memrise long enough to achieve an overall streak of 117 days…