If you have a tiddler you are editing, that contains a function definition, that makes use of a field value, and you are viewing the result in the Preview > Output most other forms of definition and wiki text are immediately reflected in the preview.
However if you change the value in a field used by a function, it will not update until you either save and reopen the tiddler, or close and reopen the preview.
Is this a bug or a feature?