The before update and after update size difference the symptom rather than the problem.

The problem seems to be something to do with lead conversion.

Here is what I am trying to test.

I have a "shadow lead" object for every lead i.e a shadow lead points to a lead.  When two leads are merged, both shadow leads will point back to the merged  lead.When this merged lead is converted to a contact, I want the two shadow leads to point to the new contact. I am setting the value of this contact on shadow leads and trying to update the two shadow leads.  The system does not want to update the SECOND shadow lead.



As I said, when I do this in UI, everything works fine - I can see the two shadow leads on the contact.  Seems like when called from Database.ConvertLead, I run into this issue.

Here is onconvert method.

trigger onconvert on Lead (after update) {

    Map<Id,Lead> convertedLeadsMap = new Map <Id,Lead> (); 
    
    Lead[] OldLeads = Trigger.old;
    Lead[] newLeads = Trigger.new;
        
    for (integer i=0; i < newLeads.size(); i++) {
        Lead newLead = newLeads[i];
        Lead oldLead = oldLeads[i];  
        if (newLead.isConverted && !oldLead.isConverted) {
            convertedLeadsMap.put(newLead.id, newLead);         
        }
    }
    
    if (convertedLeadsMap.size() == 0) return;

    Shadow_Lead__c[] ShadowLeads = [select Lead__c, Contact__c from Shadow_Lead__c where Lead__c in :convertedLeadsMap.KeySet()];
    
    for (Shadow_Lead__c sLead:ShadowLeads) {
        Lead l = convertedLeadsMap.get(sLead.Lead__c);
        if (l.ConvertedContactId != null) {
            sLead.Contact__c = l.ConvertedContactId;
        }
    }
    update ShadowLeads; //THIS CRASHES
    
}

My test method.

   static testMethod void testShadow () {
        
         Lead l2 = new Lead(FirstName = 'Test', LastName = 'Lead', Company = 'Test Company', Email = 'x2@hotmail.com');
         insert l2; 
         Shadow_Lead__c sl2 = [select  id from Shadow_Lead__c where Lead__c = :l2.id];
         Lead l3 = new Lead(FirstName = 'Test', LastName = 'Lead', Company = 'Test Company', Email = 'x3@hotmail.com');
         insert l3;
         Shadow_Lead__c sl3 = [select  id from Shadow_Lead__c where Lead__c = :l3.id];
         merge l2 l3;
           //l3 was merged and hence deleted
         integer count = [select count() from Lead where id = :l3.id];
         System.ASSERT(count == 0);
         sl2 = [select Lead__c  from Shadow_Lead__c where id = :sl2.id];
         sl3 = [select Lead__c  from Shadow_Lead__c where id = :sl3.id];
         
         System.ASSERT(sl3.Lead__c == l2.id);                
         System.ASSERT(sl2.Lead__c == l2.id);                
     
          Database.LeadConvert lc = new database.LeadConvert();
          lc.setLeadId(l2.id);
          LeadStatus convertStatus = [Select Id, MasterLabel from LeadStatus where IsConverted=true limit 1];
          lc.setConvertedStatus(convertStatus.MasterLabel);

          Database.LeadConvertResult lcr = Database.convertLead(lc); 
       }

 When the Database.convertLead method is called, I get the following exception - this is a crash at  "update ShadowLeads" statement in the trigger.

  System.DmlException: ConvertLead failed. First exception on row 0; first error: UNKNOWN_EXCEPTION, System.DmlException: Update failed. First exception on row 0 with id 00QE00000021QQUMA2; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, onconvert: execution of AfterUpdate caused by: System.DmlException: Update failed. First exception on row 1 with id a0GE0000000Zw2BMAS; first error: CANNOT_UPDATE_CONVERTED_LEAD, cannot reference converted lead: [] Trigger.onconvert: line 26, column 5: [] (System Code) External entry point: []

The before/after update symption shows up if I have before/after update trigger on ShadowLead, the before trigger has both shadow leads, the after update has only the first one. Of course, without the Shadow Lead triggers, it still crashes.