Hello,
Here are the step to reproduce:
- create an app builder page with a single line data source
- create a form with a text input field having the data source field value as default value
- set the submit button to update the data source field using the input value
Result:
- if the user changes the text input value, the data source field is correctly changed
- if the user does not change the text input value, the data source field is set to an empty string
With a dropdown input field, this is working correctly.