Never was a fan of messing with dogs while they eat. Adding, touching, subtracting...none of it is good. I will trade for a higher end item, but I totally let the dog decide and I keep my distance so it's obviously the dog's decision. That said, I do believe dogs are either pre-disposed to this behavior or they are not. The dogs that let you hug them, mess with them, poke them while they eat are not doing so because it's good training, but because it's in their DNA to act this way. Doesn't mean that dog won't tell you, at some point, he's had enough (and, it will be the dog's fault, of course). And, dogs where we did everything right and it still goes south? That dog, too, has it in their DNA to resource guard. It explains why dog A is perfectly fine with kids riding it and messing with it while it has a bone and dog B bites the kid.
All this to say, those of you who have been doing X for a long time and "it worked", don't think that doing what you did is what made the dog so accepting. And, those of you who have done everything right and are getting growling, it's probably not something you did wrong. There are things that make the behavior worse and things that can make it better, but know these dogs either have it in them, or they don't. Once a dog has shown this behavior, I completely change the interactions. When it goes south, it can be really, really bad.
It's pretty rare for a Lab to be a resource guarder. We see it in Goldens a lot. Personally, I think Goldens have a touch of anxiety most of the time, and that is why, but I could be wrong. So, I'd reach out to the breeder and let her know. It's important when dogs have temperament issues, even if they are little blips, that the breeders are aware. Be careful with hitting, yelling, etc. a dog like this. Growling is a warning. If you punish for the growl, they may stop, but they may also go straight to biting.
I'd see a board certified veterinary behaviorist right away to discuss, diagnose, and work on a training plan. This is not the kind of behavior you want a pet dog trainer helping you with.