I also used selenium recently and encountered this problem when opening a new window. I couldn't get all the handles using window_handles.
I thought about it for two days and found a lot of information. . . When I ran my script again, I found that my new window had not been fully opened (not accurate, because I don’t know how to describe this), so I could add a sleep time, or use other methods to determine the new window. Whether the window has been fully opened is more accurate than sleep

**import time**
from selenium import webdriver
from selenium.webdriver.common.keys import Keys
driver = webdriver.Firefox() # 默认的火狐浏览器
for i in range(5):
    # 这句代码相当于在浏览器窗口下按下ctrl+t打开一个新的标签页
    driver.find_element_by_tag_name('body').send_keys(Keys.CONTROL + 't')
time.sleep(10)#等待所有窗口完全打开,10秒够用了， 不好用就判断新窗口是否完全打开  
handles = driver.window_handles
print(len(handles))
print(handles)