[Python] ttk.Combobox selected text vanishes when focus is lost (clam theme, readonly)

I'm new to python and recently started coding a combobox using the "clam" UI theme. However, the selected text vanishes when focus is lost from the dropdown? It seems confined to "clam" as other UI themes such as "alt" work fine.

For example, the initial screen shows



Click the dropdown icon



Now, re-click the dropdown icon



Notice the default value apples has vanished? I was expecting it to be visible.

Example Code

import tkinter as tk, tkinter.ttk as ttk

root = tk.Tk()

style = ttk.Style(root)
style.theme_use("clam")
style.configure("TCombobox", fieldbackground="white", background="white")
style.map("TCombobox", fieldbackground=[("readonly", "white")], background=[("readonly", "white")])

cb = ttk.Combobox(root, state="readonly", values=("apples", "oranges", "bananas"))
cb.set("apples")
cb.pack(padx=40, pady=40)

root.mainloop()

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