I have a trigger which works when I am logged in a regular user.  If I am using the site guest user the same trigger executes but doesn't do everything.

In the trigger on Before Insert I create a dummy account and then try to assign to the new contact being created.  I also try to run it after insert and it just seems to skip the changing of account without error.

The account is created in either case

This works when logged in and not with Guest Account:

    if (Trigger.isBefore) {
            // get the business account record type (which is candidate) //
            RecordType NotPersonAccountRecordType = [select Id from RecordType where SobjectType = 'Account' and Name = 'Candidate'];
                        
            // create new account for one to one record to the contact, type will be the business account from above
            Account newAccount = new Account();
            newAccount.Name = 'TempAccount';
            newAccount.OwnerId = Trigger.new[0].OwnerId;
            newAccount.RecordTypeId = NotPersonAccountRecordType.Id;
            insert newAccount;    
            
            //iterate through Trigger.new and change the Contact to New Account Just Created
            for(Contact C : Trigger.new){
                c.AccountId = newAccount.Id; 
            }  
    }
 

This also work using after insert