Label behavior's difference between tkinter and ttk

[ast]

Hello

I currently migrate a GUI from tkinter to ttk and I found a problem

Here is a piece of code, with comments which explain what is 
wrong.

import tkinter as tk
import tkinter.ttk as ttk

root = tk.Tk()

BITMAP0 = """
#define zero_width 24
#define zero_height 32
static char zero_bits[] = {
0x00,0x00,0x00, 0x00,0x00,0x00, 0xf0,0x3c,0x0f, 0xf0,0x3c,0x0f,
0xf0,0x3c,0x0f, 0xf0,0x3c,0x0f, 0x00,0x00,0x00, 0x00,0x00,0x00,
0xf0,0x00,0x0f, 0xf0,0x00,0x0f, 0xf0,0x00,0x0f, 0xf0,0x00,0x0f,
0x00,0x00,0x00, 0x00,0x00,0x00, 0xf0,0x00,0x0f, 0xf0,0x00,0x0f,
0xf0,0x00,0x0f, 0xf0,0x00,0x0f, 0x00,0x00,0x00, 0x00,0x00,0x00,
0xf0,0x00,0x0f, 0xf0,0x00,0x0f, 0xf0,0x00,0x0f, 0xf0,0x00,0x0f,
0x00,0x00,0x00, 0x00,0x00,0x00, 0xf0,0x3c,0x0f, 0xf0,0x3c,0x0f,
0xf0,0x3c,0x0f, 0xf0,0x3c,0x0f, 0x00,0x00,0x00, 0x00,0x00,0x00
};
"""

img = tk.BitmapImage(data=BITMAP0, foreground='white', background='black')

# This Label comes from ttk

label = ttk.Label(root, image=img)

# This Label comes from tk. To be uncommented to test behavior'difference
# label = tk.Label(root, image=img)

label.pack()

# The graphic is not refreshed after entering following commands
# when Label comes from ttk. You have to fly over the label with 
# the mouse to get the right color.
# The graphic is immediately updated when Label comes from tk

# Enter these commands by hand, in a shell

img.config(foreground='red')
img.config(foreground='lime')
img.config(foreground='yellow')


It looks like a ttk bug, isn't it ?
I am using python 3.5.1

I tried a root.update_idletasks() to refresh the graphic
but it changed nothings.