Akuli, what tk widgets do you think are not known to tkinter?  In any case, tk menu is known to tkinter.

I cannot reproduce when running on Windows with 3.10.0b3:  Add "print(root.children)" (after add_cascade) results in {'!menu': <tkinter.Menu object .!menu>, '!menu2': <tkinter.Menu object .!menu2>}.  The names are created in tkinter.py lines 2564-2573.

I then see 'None' once and then '<tkinter.Tk object .>' indefinitely even while hovering over and clicking 'click me' and the dropdown.  If I click outside the tk box, the print returns to 'None'.

Maybe there is an OS difference in what is considered to have 'focus'.

Key '#!menu' looks like '!menu' with '#' prepended.  Someone could try changing the tkinter code referenced above and see if the change appears in the bad key.  Also check the contents of root.children.