I have a form that I direct users to with a querystring in the URL. This allows me to pre-populate a few fields with relevant data. When filling out the form, I can use the GetQueryString function to retrieve the value of a key and display that in a label if needed. But once the form is saved, that value will no longer display in the label if you're just opening it from the list (which is the norm). So I tried using the expression shown below, but it doesn't work as I'd expect. Briefly: if there's nothing in the "Quarter" field, use the value in the "Quarter" key in the querystring; otherwise, display the value in the "Quarter" field.

On form creation, the expression evaluates to:
fn-If(fn-IsNullOrEmpty(),Q3,)

After saving the form, which saves the value to a "Quarter" field, the expression evaluates to:
fn-If(fn-IsNullOrEmpty(Q3),,Q3)

So the query string is returned when the field is empty, and the field value is returned when present, but the If operation itself isn't evaluating. It's as if nested formulas aren't supported within the label. Any ideas as to why this doesn't work?