In the original Jurassic Park we saw two variations on this, one from Grant who stayed perfectly still and managed to distract the T-Rex with a flare, and the second with Malcom who failed miserably and was almost eaten.Claire almost pulls off something like Malcom in the movie. Maybe the T-Rex is just getting lazy in his old years?
I thought about this too, but the situation is very different, even though it's a call back.
The Rex has been conditioned for the past 20 years to know that the flare signifies food; so it has reason to follow where the flare goes. In the original film, Ian Malcolm tosses the flare completely out of sight of the rex, whereas Claire runs the T-Rex right into the path of Dominus. It may have been chasing Claire, but there's a much bigger, angrier obstacle right in front of it.