That's what I called security, try to add field with internalId ANYWHERE inside field name WORKS

var internalIdInputField = form.addField('custpage_xxaaaaaaaaaaxx', 'text').setLayoutType('outsideabove', 'startrow');

DOES NOT WORK

var internalIdInputField = form.addField('custpage_xxinternalIdxx', 'text').setLayoutType('outsideabove', 'startrow');

No warning , no exception , I just see blank white page instead of form. Is it normal ? When I change that one thing it shows form correctly.