The ball must be in fair territory between home and 1st/3rd. If a ball were to come from foul territory back to fair territory but beyond 1st or 3rd base it would be ruled a foul ball.
In the play yesterday the ball hit behind home plate and spun fair in front of home but obviously before 1st and 3rd without it touching a player (the catcher or batter in the box) therefore it was a fair ball. Very odd play for sure as you see that situation more with balls off the end of the bat hitting foul down the lines and spinning back into fair territory. That is why we always tell the girls to run no matter what!