Finished the German Tree, then my fluency level ...dropped?
How does that make sense? I complete all the lessons and my whole tree is golden. Then my fluency immediately drops from 67% to 66%. I can see if a few days pass and I don't do refresh lessons, but immediately?
Yes, that can happen, because .....
Duolingo's fluency % is a nonsense number.
It says nothing about your fluency in the real world.
Duolingo's Fluency % is unreliable.
Recently appeared that the Duolingo staff had manipulated this "fluency %" for testing purposes (https://www.duolingo.com/comment/22225104)
Duolingo will only bring you to an A2/B1 skill level in reading/writing and A1 (maybe A2) in listening/talking.
Info about language levels: https://en.wikipedia.org/wiki/Common_European_Framework_of_Reference_for_Languages
Read from "Common reference levels".
However, I think Duolingo's courses are excellent.
Just enjoy the courses and don't worry about Duolingo's fluency %.
It's always possible. One day you did a lot of repetitions, and now all the words you repeated then drop by one level at the same time. Ergo, your fluency drops a bit. Another problem is that Duo doesn't register all words that you repeat, in particular in the app, so the skill you repeat is golden again, but the words linked to the lexemes you did it are not leveled up, so they still appear to be overdue.
PS: If you know a bit about JSON or Python you can access these data more or less easily, there's a Duo API. Also the words list on the website gives you a rough impression.