When I run that alone I get the same effect, maybe there is something
else contributing to the previous result I saw.

focus_force did solve when testing it alone, and I think it is ok to
use focus_force when testing at least.
Here is the sample I tried:

import ttk
import Tkinter as tk

nb = ttk.Notebook()
nb.add(tk.Label(text='hi'), text='a')
nb.add(tk.Label(text='there'), text='b')

nb.pack()
nb.wait_visibility()

nb.focus_force()
print nb.select()
nb.event_generate('<Key-Right>')
print nb.select()