Why Flutter2.5 icon text field color changes from black to white?

Question

• Flutter2.2 shows the black color as icon text fields.
• Flutter2.5 shows the white color as icon text fields.

After adding IconThemeData(), I can see the black color as icon text fields in Flutter2.5

appBarTheme: xxx(
  iconTheme: IconThemeData()
)

So, I had to add this code for the icon text fields. Which source code has been changed in Flutter2.5?

Thanks for the help.