I have a business requirement to show a count of the number of completed tasks that were on a lead before it was converted. The report will be run on the account object, so I have created a field to hold this value, and am writing a trigger to count the number of completed tasks against the lead at the point of conversion, and am planning to update the field on the account with this value.
 

So far I have the following trigger, but on converting a lead, both system debugs are showing 0, and I can't work out why. I'm fine with populating the field on the account once i have the value. For now, I'm just trying to work out why the count of tasks is always 0. Both queries work when i run them in an anon block before and after the conversion respectively

trigger LeadConvert on Lead (after update, before update) {

for(Lead convertingLead:trigger.new){
  if(convertingLead.IsConverted == TRUE){
  
   id thisLeadID = convertingLead.id;
   integer i = [SELECT Count() From Task a WHERE IsClosed = True AND Type = 'Call' AND whoid = :thisLeadID]; 
    system.debug('Number of completed calls against lead: ' + i);
  
   id thisAccountID = convertingLead.ConvertedAccountId;
   integer j = [SELECT Count() From Task a WHERE IsClosed = True AND Type = 'Call' AND accountid = :thisAccountID];
   system.debug('Number of completed calls against account: ' +j);
   
  }
 
}
}

Has anyone got any ideas?

Thanks