I'm using a D5600 with an SB-800 mounted on the hot shoe aimed over my head and bouncing off the wall behind me in TTL mode. The first photo was taken at 1/60 at f/2.8, ISO100. The second photo was taken at 1/60 at f16, ISO100. Shouldn't the flash compensate for the underexposure/stopping down? I did increase my ISO but I would think that the flash should be able to compensate, especially if I am looking to avoid grain/noise in my photo.
That's 5 stops difference. It's just a matter of your flash not having enough power. Try setting it to manual 1/1 and you'll get pretty much the same thing. TTL is pretty much limited to that.