[At_Libitum]

Not quite true, if all selected books belong to the same series, the field is still blank on entry.

And if this is the underlying reason then the same should hold true for Author, Publisher, Tags, and Languages. Yet here you can type and backspace all you want without the underlying data getting cleared on the books when hitting Apply/Ok

So aleyx's claim that any field gets nulled in which you type something and then remove it before applying is not what I am seeing