How do I understand the behavior of oracle in accepting serial numbers?

We created an IO that all items need to define serial numbers with 10 characters. Upon loading the serial numbers in Miscellaneous Transaction, oracle won't accept the serials unless we put "-" before the last 4 digits.

In example serial number 0123456789, oracle won't accept this unless I make it 012345-6789. But like I said half of the data I loaded oracle accepted it in same format, example serial number 0987654321. There are some that oracle does not accept the serials with dash in the last 4 digits and the original serial number, that's why we put the dash in the 5th digit of the serial number.