I have a really basic trigger on the EmailMessage object that updates the Case Status when an incoming email is inserted. The trigger works just fine in testing and in my test class the System.assert passes. However, the trigger has 0% test coverage and I am not able to deploy to production.  Are there limitations with this object that I'm not aware of? Any idea why this is happening?

trigger updateCaseStatus on EmailMessage (before insert) {

    List<String> applicableStatus = new String[]{
        	'Support Agent Working - Resp Provided',
            'Resolved - Waiting for Customer Approval',
            'Waiting for Customer'};
    
    Set<Id> caseIds = new Set<Id>();
    
    for (EmailMessage em : Trigger.new)
    {
        // if it is an inbound email
        if (em.Incoming)
        {
            // add Case Id to caseIds set
            caseIds.add(em.ParentId);
        }
    }
    
    // query for Cases to update
    List<Case> casesToUpdate = [SELECT Id, Status FROM Case 
                                WHERE Id IN :caseIds AND Status IN :applicableStatus];
    
    if (!casesToUpdate.isEmpty())
    {
        for (Case c : casesToUpdate)
        {
            c.Status = 'Customer Response Provided';
        }
        update casesToUpdate;
    }    
}

@isTest
private class updateCaseStatus_Test{

    static void testInboundEmail() {
        // create test case
        Case testCase = new Case(
                Subject = 'Test case01',
                Priority = 'P1 - Emergency');
        testCase.Status = 'Waiting for Customer';
    	insert testCase;
        // System.debug('testCase: ' + testCase);
        
        // create test email message
        EmailMessage em = new EmailMessage(
        	Incoming = TRUE,
            FromAddress = 'someone@somewhere.com', 
            ToAddress= 'someone@nutanix.com', 
            Subject = 'Test email', 
            TextBody = 'Test',
            ParentId = testCase.Id);
        
        Test.startTest();
        insert em;
        // System.debug('em: ' + em);
        
        // query and system assert
        Case c = [SELECT Id, Status FROM Case WHERE Id = :testCase.Id LIMIT 1];
        System.assert(c.Status == 'Customer Response Provided');
        Test.stopTest();
    }
}