Did Canelo score a knockdown in the 9th round?

No. Canelo did not step on his foot there. There was no foot collision in the exchange that sent Lara down in the 9th. Canelo landed the jab over Lara's right jab, and skillfully planted his left foot outside of Lara's right foot. Lara freely moved his right foot out of the way.

There was another time in the fight where a foot was stepped on, but not at this sequence. Please take a look at the footage again.

 

He chose to jump away, after being hit flush by the jab. He wasn't quick enough to avoid that jab. He tried to play off being hit by the jab by making it look like he slipped it and danced away. Pure boxers do this trick all the time. In fact Mayweather did it vs Canelo as well. Canelo would land a jab on Mayweather as Mayweather was trying to slip it. Only Maywather has better footwork than Lara so he was able to make it look like he slipped the jab when it actually landed glancingly.

That jab had Lara off balance when he "chose" to jump away. His feet gave out from under him as he jumped away. If you try to jump away as you are being hit by a punch, there's a good chance you are going to fall down.

 

He's able to roll with punches and not lose his balance like Lara did. Because he has better footwork than Lara. Canelo landed the same type of jab on Mayweather, but he slipped it, or partially slipped it, and reduced it's impact. Lara didn't have the speed or the footwork to avoid Canelo's jab like Mayweather did.