This is related to how the CMOS chips are scanned. Often referred to as “Rolling Shutter.” The flash happened mid scan so part lands on one frame and part lands on the next.
While it’s “different” that what one would get with CCD, I’m not sure if “different” automatically makes it worse. None of my clients has ever commented on it. I’m sure they notice it but I suspect they don’t see it as any more annoying than a flash.
I’ve seen much debate over how the look is impacted by frame rate and shutter speed. They don’t impact the scan rate of the chips themselves but can impact how the image looks on a given frame. Changing frame rate and shutter speed have other aesthetic impacts though that one has to consider.
BTW I’ve shot fireworks many times and there’s no visible artifacts. The issue becomes obvious when the flash (or other strobe . . . like the flashing light of a police car bubble) fills most or all of the frame which makes the split scan more likely.