The Additional Roles setting is for adding an additional role to the existing role for a user, so that the user has multiple roles.
If you only want one role, and to change which single role is assigned to the user, then this setting is not for you.
You should uncheck any additional roles you may have already selected, and then use the normal role selector to choose the one role you want to assign to the user.
In your screenshot the user has the Subscriber role set, and additionally has the custom Tour Guide role assigned.
If you only want the user to be a Tour Guide, you should uncheck Tour Guide from the Additional Roles section and then specify Tour Guide in the Role dropdown.
I watched the video, but I'm not sure what I'm supposed to get from it, it looks like things are working as expected, I couldn't spot the problem.
What changed between 2.6 and 2.8 was that support for multiple roles was added. If you don't need to assign multiple roles to a user, you need never expand that "I want to add more roles to this user" section, and just use the dropdown in the same way as before.
Is the problem that you don't want the support for multiple roles at all? You want to disable it?