It seems to sort of be 'one key behind'. For example, if I need two fields to match each other, I could type:

"1234" in the first field and "1234" in the second field, but it won't register as matching until I press another key! So if I type "12345" in the second field it will then register as two matching fields

Do I need to use a different event handler to fix this?

probably onkeyup or onchange