Practice/peeking/spaced repetition buggy?
After looking in the vocabulary section I could see that the word 'allgemein' had been studied 3 months ago and was in my long term memory. Then, I practiced it and had a peek. Back to the vocabulary section the word was still considered to be in my long term memory whereas it should have been put back to the short-term memory one!
I saw this behavior with other words while practicing.