Is this a red flag?

A neighbor just got a puppy -- a 5 month old lab/golden mix from a shelter. ABSOLUTELY BEAUTIFUL puppy! The day after she got her (a little over a week ago), I walked by her house with Micky and saw them in the yard (a fenced yard --- wrought-iron, not a privacy fence). Micky and I approached so Micky and the puppy could meet through the fence. Micky approached excitedly doing play bows and wagging his tail. The puppy stood stock-still and looked stressed so we immediately backed away. My neighbor told me that she had been stressed since she brought her home (it had only been one day).

Yesterday we walked by again and saw the puppy in the yard (my neighbor was not out there with her). Again Micky and I approached to greet her -- we approached very slowly because of how stressed she was last time. This puppy looked frozen she stood so still... lifted her lips and gave a continuous low growl. Micky and I left immediately.

Is it unusual for a puppy that age to react like this to other dogs?