Hi Andrew. As you have seen the dog confidently “perform” with handlers in whom trust and structure had been established, it is likely a possibility that you will see the same once you have an established structure in your pack. In the former environment, the “leader/alpha” was established and the dog was able to perform as expected. If the dog is not yet sure, or assured who is alpha I believe it it very normal to see some hesitancy and unsureness as there is ambiguity as to who is alpha and what each member of the pack’s responsibility is.
Establishing yourself as alpha is paramount and takes a little time. There can be no doubt. You are the provider, protector, and leader. When roles are defined and routinely established your dog is set up to perform what its role is. Being in a new pack is certainly a different experience for your dog. Dogs thrive in structure and while pack change is not an issue, its the structure within the pack that has the greatest impact on a successful change. In short, a dog changing packs looks to see who the leader is. Leadership is not synonymous with dominance. A dog that is dominated is generally not reliable. A dog that defers to its leader is reliable and able to maximize its potential.
With time, structure, structure, and structure you will successfully establish your role and I suspect “nervousness” will be replaced by confidence and the awesome bond will facilitate the working confidence you witnessed prior to purchase. With structure confidence is possible.
There are many subtle things you can do to affirm your leadership which serves to define roles which I believe will boost canine confidence. You can either opt to train/desensitize in various situations or work on structure that supports confidence which is then present in all situations, new or old. Enjoy the great journey you are on – its awesome!!