oh I know exactly what you're talking about!
Even in the show Wildfire, they use different horses all the time. And I always catch onto it. Like they'll be racing a horse that has no facial markings, but when you get a close up of the horse in the race, there's magically a start on his forehead. Or they'll use a bay stocky i'm going to guess qh, and then he'll be running and look like a thin TB.
I just find it hilarious.