Hey Constanza,
Did you try assigning the Survey entity as Completed= false as default?
from the attribute properties. This way, your attribute will be set to false as soon as it is created. and then you can change it to true through a microflow when you want.
Let me know if you face any issues,
Hope it helps!
So for the seek of sharing the solution:
I delete the second association, just use one (one to many), and in the microflow, specifically in the create object questionary, I add another element to connect the questionarie with the survey, and assign the value of the survey object created directly before, and last left the questionarie create object tab with No in refresh in client.