If you choose a specific invoice number from the F2 (by Invoice Number) list, it will display the oldest invoice produced from the associcated Sales order instead of the invoice number that you chose.
 
When you display the F2 list by Sales order, it properly groups all invoices produced by a given sales order.  No matter which invoice you pick for a given Sales order, you are presented with the oldest invoice.
 
If you use the F2 list to identifiy a specific invoice number, and then exit the F2 list and enter the invoice number, the correct invoice is displayed.

I know I have confirmed this myself in the past but I just tried, both in classic DBA and Evo and I can't make it not work now, neither can the programmer.  We have made some unrelated changes to the lookup library and recompiled so maybe it is finally fixed.  Kevin, I will send you what I have and you can let me know.