When you have set a property (of an expression field) as "Use an expression" and you've set the expression, you can no longer change the expression. Even worse, if you try, you get an empty (thus deleted) expression field.
Problem has 2 sides:
- When you've set the property to "Use an expression", JSS does not remember this choice and shows an empty text-box.
- When you reselect the "Use an expression", you are presented with an empty expression-box.
Reproducible since JSS 5.5 and later. Did not check earlier versions.
Maarten @ Yesplan