After trial and error, I believe the behavior is different than I was expecting. I had two boolean values defined in user attributes. These values were not added to the user attributes of the user flow but added to the application claims. My assumption was the values would be set to false in this case but they are not.
In short, it is as simple as if a claim does not have a value then it will not be returned. So a boolean claim will then have three possible values: true, false, missing.
I tested this with Postcode and it has the same behavior