It seems to be some weird internal mismatch. Code modified from https://wiki.tcl-lang.org/page/Tk+differences+on+Mac+OS+X (see following) suggests that dark mode is not enabled for the window (and yet we are getting white foreground colours)

import tkinter as tk
root = tk.Tk()
print(root.tk.call("tk::unsupported::MacWindowStyle", "isdark", "."))

I presume this is a MacOS 11 issue. I will look into raising this with the Tk team.