Are you sure that Lefty was growling at Jeff? or some unseen noise in the distance? The reason I am asking is that sometimes Leo does this, but it always turns out to be something outside that he is growling at (i.e. the mailman, the ups truck, someone walking past the house) and when they go away he just looks at me like "see Mom - I made them go away".
It almost sounds like Lefty was growling and something else and then when Jeff moved got refocused on the petting.

I do not ever think it is permissible for a dog to growl at their parent - that is not to say you need to get rid of the dog, but you might need to put it on the "Nothing in Life is Free" program.