It's not ideal, but could be worse. The old decking gave way. Whether the roofers saw that and covered it up or it gave through when the air nailer got it is another matter. Most roofers would probably keep going till the job was done and address isolated issues like this after.

If decking repairs or a warranty were in your contract then they should make it right. If you don't have a contract then it's at the discretion of the roofers if they want to do anything about it.

Over the life of the roof that spot may wear prematurely or give way if any weight hits it. It's under a vent so there is probably a metal flange over it. So it might hold up. Time will tell.

It's certainly fixable. To do it the right way you need to take it all the way down to the decking and patch in a new board. Certainly doable, but kind of a pita.