[SalesForce] Sandbox modifies subject on email sent from trigger

Hope the below code helps!

trigger TaskSubject on Task (after update, after insert) {

set<id> accountIdSet = new set<id>();
//capture all the subject names in a set
//set<string> subjectName = new set<string>{'Call EJF','Email Centurion','Initiate Merion'};
map<id, list<opportunity>> opptyMap = new map<id, list<opportunity>>();
list<opportunity> updateOppList = new list<opportunity>();

//get those tasks where the tasks are related to account. Capture the account IDs in a set.
for(task t: trigger.new){
            string tempId = string.valueOf(t.WhatId);
    if(tempId.startsWith('001')){
        accountIdSet.add(t.whatId);
    }
}

//If we have any account IDs then get the associated opportuity and store the one account to many opportunity in a map<id, list<opportunity>>
if(accountIdSet.size()>0){
    for(opportunity opp:[select Name, stageName, accountid from opportunity where accountid IN :accountIdSet]){
        if(opptyMap.containsKey(opp.accountId)){
            list<opportunity> oppList = opptyMap.get(opp.accountId);
            oppList.add(opp);
            opptyMap.put(opp.accountId, oppList);
        }else{
            list<opportunity> newOppList = new list<opportunity>();
            newOppList.add(opp);
            opptyMap.put(opp.accountId, newOppList);
        }
    }
}

//If we get the opportunities then change the stage.
if(Trigger.isUpdate && opptyMap.size()>0){
    for(task t: trigger.new){

 //Check if the subject of the task is one among the set 
 //and also confirm that the subject has been changed.

        if(t.subject != trigger.oldMap.get(t.id).subject && opptyMap.containsKey(t.whatId)){

  //iterate thru the list of the opportunity associated with that account 
  //and check which of the opportunity contains the task subject in the 
   //opportunity name and update the stage to qualification

            for(opportunity oppty: opptyMap.get(t.whatId)){
                if(oppty.name.contains(t.subject)){
                    oppty.stageName='Qualification';
                    updateOppList.add(oppty);
                }
            }
        }
    }
}
if(Trigger.isInsert && opptyMap.size()>0){
     for(task t: trigger.new){
            if(opptyMap.containsKey(t.whatId)){
                for(opportunity oppty: opptyMap.get(t.whatId)){
                   if(oppty.name.contains(t.subject)){
                      oppty.stageName='Qualification';
                      updateOppList.add(oppty);
                   }
               }
            }
        }
    }
//update the oppties
if(updateOppList.size()>0){
    update updateOppList;
}
}

EDIT:
This code is working. I followed the below flow: Create an account(test account) --> Create a task for this account (test account) and make sure the subject is not 'Initiate Merion'.
create an opportunity (make sure the stage is not qualification) for the same account, name it 'test oppty Initiate Merion'.
Now edit the task created, change the subject to 'Initiate Merion'. You can now see that the Opportunity stage has been changed to 'Qualification'.

EDIT 2
Added an after insert event. Also the one more code block to handle the insert and update events seperately

EDIT 3 Task subject need not necessarily be from a set of predefined strings. As long as task subject is part of Opp name, stage must be updated

I believe you are subject to this when a task is created as a result of Send an email:

https://help.salesforce.com/apex/HTViewSolution?id=000213837&language=en_US

Workflow rule not triggered after clicking on send e-mail button from the object, in spite of the completion of the newly created task.

This is an expected behaviour. Please find the explanation below

• When Salesforce sends an email from a record, it accesses the email author page (Javascript page)
• When send is clicked, email is sent from the object. An internal workflow then adds the email to the activity history as a completed task.
• This action of adding the completed task does not trigger additional workflow (i.e. your task workflow) because it was generated from an internal workflow. Similar behaviour is seen for Process builder on Task that has email as a scheduled action.

This behaviour concerns the standard email scenario, where in email is stored in a task. When Enhanced Email is enabled, both a Task and an Email Message are created, it is possible to trigger a Workflow or Process Builder of that record. Considerations for Setting Up Enhanced Email

Mass mail, does not use Enhanced Email, only a task, and as such, you cannot trigger a workflow / Process on those records.