Yes - it all looks fine. I enter ISBNs individually, using CueCat, they all appear in the window, I click Add, the blue bar appears, there are 0 not found, so I click OK and just one title is added. So far, it has always been either the first or last in the list.
Could you please tell us your preference for searching, what site do you have set on the search window and if it's cascade which ones you have on in the preferences. As well as the sort order in that same window. Maybe two or three of the ISBN as well so we can try with the same information just in case. Thank you.