The next thing I would check is, for records with a particular debtor number, are the values truly identical. Numbers of zeros, spaces etc
I'd make a query based on SCA_T, pick a value from tmpDebtor, then copy and paste it as a parameter to see if actually pulls back what I'm expecting.
Another thing to look for would be any records things like o instead of 0.
If you need another pair of eyes you could always just extract the debtor number by itself into a table, which wouldn't cause any confidentiality problems I would have thought, to post. Then give an example of the values giving problems.