I'm not sure why that would happen if you were at 1/1600th. I am wondering if you set the camera for 1/1600th but really got a much slower shutter speed because the camera wanted to synch with the flash. My Sony V3 drops to 1/60th of a second when the external flash is operating because it relies on the speed of the flash to freeze motion.
I expect that you were pretty far away from the gymnasium floor while shooting your granddaughter and probably using the excellent zoom capability of the 828.
If that was the case, then the flash range was probably exceeded and, if the camera defaulted to its flash synch speed then you would have a darker picture at a slower shutter speed...and blur.
Also, using the zoom, you might have run into camera shake which is, of course, worse the more you zoom out. If nothing else, camera shake would have contributed to blurry shots (especially if shutter speed was reduced).
Can you give us a sample and the EXIF readings from the file? We could then have a better idea what the camera was actually doing.