I am trying to set a visibility rule on the 'Cancel / Exit' button in the Function Bar, but any rules I create appear to be ignored. If I set the rule to return false, the button is still visible in the form.
Is there some other logic governing the visibility of these buttons? How can I create a dynamic visibility rule for them?
As a form builder you usually won't have access to modify it. Are you in the form or template?
Guessing this is of no use Chris ? Visibility Rules
As noted, the Cancel / Exit button visibility is controlled by the Template, and not by adding a Visibility Rule to the form.
As form designer, you CAN turn the Cancel / Exit feature on or off. Click the Form Options button in the Maestro toolbar, expand the User Experience section, then click the Include Cancel Feature item. There you can select/deselect the checkbox to include the cancel feature.