I have enabled Email-to-case in my org and followed steps below.

Step1: Customer sent an email for the routing address, and case is getting created in Salesforce

Step2: When the Salesforce User responds to the Email from case Quick action then it is going to customer and i can see both the Step1 thread and Step2 Information in the quick action.

Till here everything is as Excepted.

Step3: Now when the customer is responding to the same email , I should be able to create new Case in Salesforce instead I can see that the Step3 response related information in the Parent Case Activity.

Till Step3 results

I have enabled the Thread details in email-to-case settings.

Insert email threading token in email subject and Insert email threading token in email body.

Excepted Result : When customer is responding to the reply email i should be able to create new case which will have Reference_Case_Details__C (Lookup to case) and populate the old case record into Reference_Case_Details__C.

For Populating the old values i have written the trigger which is stated below, but some how this is not working, Can anyone please suggest me the appropriate changes to be done.

code :

trigger MI_EmailMessageTrigger on EmailMessage (after insert) {
string fetchString = 'ReferenceID::';
string pastCaseNum = '';
for(EmailMessage em: Trigger.New){
    if(em.TextBody != null && em.TextBody.contains(fetchString) && em.Incoming && em.ParentId != null){

    List<String> words = em.TextBody.split(' ');
    for (Integer i = 0; i < words.size(); i++) {
        if (words.get(i).equals(fetchString)) {
            if (i < words.size() - 1) {
                pastCaseNum = words.get(i + 1);
                if(pastCaseNum != null){
                    break; 
                }
            } 
        }
    }
        for(case cs: [SELECT id,Reference_Case__c,description  from case where id =:em.ParentId]){
          cs.Reference_Case__c  =  pastCaseNum;
        /update cs;
        }
    }
}
}

Tried with all the settings to be checked for creating new case and for the trigger part have created new EmailMessage record from anonymous window and tired to debug