He is probably unhappy with the current GK situation. My guess Flick wants to see Tek in action and test if Tek can be a difference maker when necessary. Pena has a contract ending in 2026. But my guess is that he is going to renew like Lunin did. There is not shortage of good goalkeepers on the market and it is difficult to see where Pena could go because all clubs in major leagues are fully staffed in the GK department.