Sorry... I did not read far enough down your post otherwise that would have been an obvious answer. If you create another group and apply the auto user creation to that new group does it change anything or does it keep it public? Just making sure that the new test group that you create actually gets populated on the DT side.
Probably not related but after an upgrade, I noticed some issues that required me to "repopulate" certain configuration fields in DT, even though it was displaying my configuration correctly. After repopulating and re-applying those fields, things began working.
First, make sure the "Registered" group is selected for the default user group assignment in configuration and re-save configuration.
Second, any user group selected inside of the event next to the auto-user creation area will override the global setting. So if you want it to use what is set globally, make sure no group is selected within the event then re-save the event.