When submitting a new feature in ArcGIS Field Maps (iOS) with a field which does not allow null is hidden with a conditional visibility expression even if the field already has a default value defined under Templates, the following error message is returned, "Feature isn't valid."
Set the default value of the no null value field to hide using calculated expressions.
Remove the conditional visibility expression on the null value field fixes the issue and the default value set up under the Template tab gets auto-populated to the form in ArcGIS Field Maps.