Iza is the most common short version of Izabela in Poland
It is. But I think 'Bela' was preferred in the 19th or early 20th century When you divide the name, you will have "Iza-bela", and they now prefer the first part as a shortned form, while more than a hundred years ago they chose the final part. Another thing is that the name wasn't popular among the so-called common people at that time.
Such preferences change over time. For example, I've watched a very interesting lecture of the archeologist Przemysław Urbańczyk on the times of Bolesław Chrobry recently, and the lecturer noticed, comparing the iconography on banknotes and coins, that the king was portrayed en face in the times of the PRL, whereas before the WW II and today he is portrayed in profile.