First of all, the full process and nature of Hollows turning into Arrancar is yet to be revealed, so we can only fill in the blanks based on what we know so far. Now, ff the Hollow dies or is cleansed while the mask is damaged, it's a moot point. While the mask is allowed to break, it needs to be removed completely. If it's merely cracked, of course nothing will happen and it will probably heal. And I'm pretty sure the "Hollow" you're talking about is Demoura Zodd, who was, in fact, an Arrancar. And it's still unknown what exactly happens if an Arrancars mask is broken. Or, at least, what is supposed to happen.