Customer ABN missing

When I enter a customer I always put their ABN in on the Selling Details tab.  However, when I go back into them, some are missing.  We do a transfer to/from an external hiring program but it also includes their ABN and this is the way that MYOB knows that it is the same customer so it shouldn't be that it is deleting them on transfer.  Not all customers have lost their ABN so it seems very sporadic.