Is this a glitch in the system?
I have a 4 day streak but I missed two days of using Duolingo before yesterday and I don't use streak freeze? Is this a bug?
It might have something to do with Duolingo's database problems on the fifteenth: https://status.duolingo.com
I believe that after experiencing this problem, they tried to make sure that people did not lose streaks because of it. As such, they may have restored some streaks lost on or around the fifteenth.
And I'm fairly sure that some people's streak freezes (not everyone's) did not get used on the fifteenth, although they were unable to use Duolingo.