I don’t think it’s a bug, as I think it’s been intentionally designed like this, because it’s easy, but offline activity (such as listening offline) records the activity only on the day when you reconnect to the Internet.
For example, Sep 1 = 2 hours listening (offline), Sep 2 = 1 hour listening (offline), Sep 3 = 1 hour listening (offline), Sep 4 = 0 hours of listening, but reconnected to the Internet. LingQ will record 4 hours of listening done on Sep 4, when in reality I did no listening on this day.
For overall statistics, this works fine. But if you are trying to keep a streak, it does not work.
The other month, I had one day away from Internet access, but I still did my listening practice. Obviously, LingQ thinks I missed a day and so I lost my streak. The problem (this is actually a bug) was that because it recorded activity on the day I reconnected, I now had a one day streak and it would not let me press the ‘repair it!’ button, because it said “The streak is not broken.” See screenshot.