They have proved themselves in every term in the Europa League competition but they have failed to do so in their own respective League. They have played against teams that are stronger compared to themselves in the Europa League and won against them and on the other hand, they have played against somewhat weaker teams compared to themselves and lost in the Bundesliga. I really hope that they will be able to do better in the next season in Bundesliga.
This may have something to do with their focus. This season they are focused on the Europa League and that is very good for them to make history.
On the other hand, they are playing badly in the Bundesliga because their focus is not there and as long as they are not at the bottom and near the relegation zone is good for them.
I think this strategy is the same as Villarreal in the Champions League although the results of the two clubs are different at the end