After running `start` on an indeterminate Progressbar, it animates to one side, goes back to the other and then right before it reaches the end it disappears. I've attached a sample script below and a screen recording. This is running on macOS 11.3 and python 3.9.6 with the tkinter version that is packaged with the 3.9.6 installer. 


```
from tkinter import *
from tkinter.ttk import *
import time

window = Tk()
window.title('Test')
window.geometry('400x250+1000+300')


pb = Progressbar(window, orient=HORIZONTAL, length=100, mode='indeterminate')
pb.pack(expand=True)

Button(window, text='Start', command=pb.start).pack()

window.mainloop()
```