I'm not surprised that the voids aren't present in all, and that's what I was driving at earlier.
I also wouldn't discount the possibility that they were left by carbides out of hand. As you say O1 *shouldn't* have that problem, but it seems clear that these chisels are not as they should be, so there's no inherent contradiction. As I said in my initial reply, this screams "processing issue". It's possible the billet had internal voids, though more likely IMO that something happened to screw up the microstructure, leading to coarse grain/carbides.