Understanding When Field Default Value Expressions Are Evaluated
A default value expression provides you the ability to dynamically calculate the initial value of a field in a newly-created row.
If you configure a default value expression for a field, it is evaluated only when a new row is created. The expression is not evaluated at any other time.
If your use case requires the value of a field to change automatically when the value of one or more other fields is changed, see Deriving Values of a Field When Other Fields Change Value.