I first noticed this behavior around the beginning of July along with others that work with lists extensively. At the time, I assumed it to be a new, secret rate-limit (or at least was not updated in the docs, I know docs can trail behind). But, glad to see this thread since it is still happening and the docs have not been updated, if indeed it is a rate limit of sorts.
I can confirm that the call behaves differently with a identical set of user IDs, which was why I assumed it was some type of limiting.
I can also confirm that once it starts happening (not all users added in the create_all) then attempts in the native Twitter Apps/site to add a member to a list also fail, again pointing to some limit on the user in the number of memberships that can be created in a given period. The error message in the Twitter web site is “Your account may not be allowed to perform this action. Please refresh the page and try again.”