When you use spikes and zones and plant stakes there are going to be hot spots in the container. Roots not specialized to be in that hot and sometimes wild pH area of the container can burn and even prune themselves off. On young plants without a whole lot of leaves to be able to express this, you can get severe burning like this on one or two leaves, and then it goes away. That leaf ends up holding on and the problem seemingly goes away. The plant simply adapted to its environment the same as it would do finding any other obstacle.
Your clue that this wasn't a critical problem is that it only affected a couple of leaves before it didn't replicate on the new growth. An interesting experiment sometime would be to take a healthy tomato plant and chop off 1/3 of a side of its rootball just to see what happens. Something similar to what you have just seen will be the result, and then the vegging plant will just keep on keeping on.