The wood is split from the screws. Holes should have been pre-drilled that close to the edge. In the middle of a board it’s not so apt to split. The big chunk that’s broke off might be because of the screw split as well or maybe it was just a bad board.
I’d make them fix it. This isn’t normal. Since stairs only really need good compressive, as opposed to shear strength, that wood can probably be epoxied back together, clamped until cured, and reattached. 1/2
@DietCanesSauce Also, if there’s not too many steps you might want to have the tops of the stringers covered with a waterproof membrane. This would require taking all the treads off so maybe not feasible. Water can sit on stairs for long periods and make its way into the wood which will eventually rot. The membrane reduces that penetration. A lot. You only put it on the top horizontal surfaces, basically under the treads. Never on the bottom surfaces.