Fearful of People (in some situations)

My partner and I adopted an American Pitbull terrier mix at 6 months old. She'll be a year old in April. She is the sweetest dog in the world. She LOVES other dogs but has some trouble with people. When we're at the dog park she'll go up to greet people and if you have a dog with you she is fine but she'll back away and bark and quietly growl when people come in our house, no matter how many times she's met you. She'll also do this in public places but is pickier and sometimes it's people who approach her incorrectly, reach for the top of her head, stare her in the eyes or are generally sketchy looking people. 

At home we try and meet people outside and all walk in together and we give our guests treats and tell them to ignore her. This seems to work after a few minutes and she'll settle down but if a guest gets up or makes any loud noises she might bark or growl under her breath.
Whenever she meets someone she always looks at their faces and since she's
So cute I know it's hard not to look back at her. Her litter was raised to be bait dogs as far as we know but we get together to play with her sisters all the time and they don't have any fear problems at all.
I guess I'm just wondering if we're doing everything right or if anyone has any suggestions. She's very smart and well-trained. Thanks!

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