That void is rather large, so it may have been there in manufacture. However, I have seen voids in many bullets which I've sectioned, including the typically accurate Nosler BT. I have also noticed however, that sometimes the core is re-solidifying after I face off the surface and that I have created voids by my method of sectioned them.
FWIW, and this is not a suggestion so do as you wish, but I often drill an appropriately smaller hole in a 2 x 4 for the bullet I plan to section, so that it fits snugly when it is pressed in. Then I carefully, (sometimes), cut it nearly in half with a good blade on a table saw. One can then use a stationary sander to slowly sand the bullet as far as desired. As I suggested earlier however, getting too aggressive may mean you'll melt any lead the bullet may contain.
