The change was intentional as the result of the new feature 4700857, where the default locale is now derived from the DISPLAY locale. On Windows, it is derived from the user's UI Language.
However, the Windows Control Panel only allows users to select "language" without any region information for the UI Language in its selection menu. For example there is only one selection "English" for all English locales. To remedy this situation, if the language portion of UI language and the user locale (the "Format" locale) are the same, UI Language's region information will now be replaced with the one in the "Format" locale.