Can't edit data of ScriptableObject

Hi!

This is my setup: I have an instance "Level1" of a ScriptableObject "Level", whereas Level1 stores all the data of my current level. Level1 is assigned in the inspector to another script that takes the data of Level1 and builds a level out of it:

This is the problem: Whenever I start Unity anew, I can edit all the values of level1 perfectly fine. However, after I started the game once, I cannot edit anything anymore: You see that the text changes right back to the original. I then have to restart Unity. Note that there are no logs in the console. There is also no CustomEditor script handling "Level" (I used to have one though). The Unity version is 2019.3.14f1.

Thank you for any help / suggestions!