I think it was a sobering reality check and a signal that they need to continue working hard to repeat their success and nothing will be given to them for free. There was almost something magical about their undefeated streak, especially given the 3 recent 90+ minute equalisers, which could create an impression that there's some higher power behind them preventing them from losing. They might have believed it a little bit too much and got a cold shower.
The approach to the game was quite relaxing; it looks like they underestimated their opponents and were shocked when the first goal entered, the second goal entered, and then they could not score before halftime. They enter the second half believing that that same magical force that has been helping them equalize and win games will have them equalize the two goals and lead them to a victory. It was not their day, Ademola lookman banged their net for the third time, shattering their dreams.
This is the first defeat for the team this season, and for some players, it is their first since joining the club. They will be proud of their achievements this season, but even with Xabi Alonso still as coach next season, I do not think they will be able to retain their Bundesliga glory.