Rus Mastery Forester is unlocking bugged

I tried unlocking the Rus Mastery “Forester” (second mastery) several times. I tried building 4 pairs of lumber mills next to a wooden castle each as well as 4 lumber mills next to one wooden, nothing works. Am I doing something wrong or is the mastery just bugged yet? Sorry if the English words are not correct, I am playing in German. I hope you know what I mean.

Hello, can you let me know if you’re playing from a saved game, or new?

I started a new 1vs1 game against the KI twice. Didn’t work woth of the times. Thanks for working on this!