For disclosure, what I know is talking to a friend who is passionate about the breed, not direct experience.
Based on conversations I had with her, what you describe doesn’t surprise me. She said they tend to bond pretty tightly within their pack but take a while to let anyone else in–so you are part of her pack, but she will distrust anyone outside it. They haven’t been bred for centuries like so many other breeds to see extend the pack acceptance to most people/dogs generally. Obviously there are individual differences, and nurture/socialization might help overcome that, but she did mention it as something she’s seen with her dogs and the ones she’s rescued/fostered.
Also–is your dog an only dog? Because that pack mentality is pretty strong, she said they often do better in groups. It might (total conjecture here) help her out if she had another dog to bond to, so she wasn’t alone when you left on longer trips. And if that dog was a socially outgoing dog that loved people, it might help her learn to welcome others more easily than she does now.