I've just come across the same problem.

For future reference, adding the following code before using a Treeview widget will fix the problem:

def fixed_map(option):
    # Fix for setting text colour for Tkinter 8.6.9
    # From: https://core.tcl.tk/tk/info/509cafafae
    #
    # Returns the style map for 'option' with any styles starting with
    # ('!disabled', '!selected', ...) filtered out.

    # style.map() returns an empty list for missing options, so this
    # should be future-safe.
    return [elm for elm in style.map('Treeview', query_opt=option) if
      elm[:2] != ('!disabled', '!selected')]

style = ttk.Style()
style.map('Treeview', foreground=fixed_map('foreground'),
  background=fixed_map('background'))