@Woo and I looked at this together. The default_channel_invitation_preference is only utilized when a user is created. Once the user is created, changing the value of default_channel_invitation_preference does not retroactively change the preference of existing users, it will only set the new value for user created after the change.
I understand that in the documentation, that is not clear at all. I’ll work with our Documentation team to help clear that up.