Scriptrunner Behaviour Not firing when custom field value is changed in Portal Form

[Doods Perea]

I have 2 custom fields in Customer Portal form:

1. Audience - Multi-select field
2. Audience (Not in selection) - text field; optional on initial page load

When "Audience" field value contains "Not in selection", the "Audience (Not in selection)" field becomes Required.

When "Not in selection" value is removed, the "Audience (Not in selection)" field is expected to be Optional.  However, this field remains required.   Here is my Behaviour code:

import com.atlassian.jira.component.ComponentAccessor

def txtAudienceNiSel = getFieldByName("Audience (Not in selection)")
def selAudience = getFieldById(getFieldChanged())
def selectedOption = selAudience.getValue() as String
def isNiSSelected = selectedOption.contains("Not in selection")

if (isNiSSelected) {
 if (selAudience){
 txtAudienceNiSel.setRequired(isNiSSelected)
 }

What could be wrong with this?  Thanks in advance for your help. 

@A, @Mark McCormack (Adaptavist) - can you please help here?  Thank you.