Wrong Regional Formatting on CalendarDatePicker in packaged app vs unpackaged AppSDK 1.6

[Zulu-Inuoe]

Describe the bug

After upgrading to AppSDK 1.6, I found that CalendarDatePicker is now correctly rendering dates according to region settings, but only in an unpackaged app.
In a Packaged app, the regional formatting is incorrectly being pulled from the UI display language, rather than the regional setting.

• System language is English (United States)
• Country/Region is United States
• Regional settings are German (Germany)
• I've overridden the short date format to YYYY-MM-dd
  

In a sample app with only a <CalendarDatePicker />, I see different results when running the app packaged vs unpackaged:

Unpackaged (correct):

Packaged (incorrect):

You can see this incorrect behaviour in the Gallery app, as well
I can reproduce this behaviour both on Windows 10 and Windows 11

Steps to reproduce the bug

Create a CalendarDatePicker

Expected behavior

The picker should display dates in the user's regional formatting (YYYY-MM-dd in this case), whether packaged or unpackaged

Screenshots

Unpackaged (correct):

Packaged (incorrect):

NuGet package version

WinUI 3 - Windows App SDK 1.6.0: 1.6.240829007

Windows version

Windows 11 (22H2): Build 22621, Windows 10 (21H2): Build 19044

Additional context

No response