Regional data preferences not honored when signing out

I have set my regional preferences to show time in 24 hour format.
Ie.
- short time shows as 16:15
- long time shows as 16:15:33
When I lock my screen (Windows + L), the time is shown correctly in 24 hour format on the lock screen.
When I sign out of Windows, however, and I'm prompted to login, it's shown in 12 hour format (04:15).
How I can change this setting so that the time is always shown in 24 hour format on the sign-in screen.
Note that I'm using a local user account, not Microsoft account.

spunk said:

Is this a work computer? Is your computer part of an Active Directory Domain? If so, then the Time is set by the Domain Controller.

No, it is a home PC. There is no domain controller.

spunk said:

Boot into Setup (Bios) set the time here. It should give the option to change it to 24 hour time.
You can also download a Bios update from the computer manufacturers web site.

No. This is a display issue on the Windows login screen. It has nothing to do with the BIOS.

This is likely controlled by a registry key somewhere. If there is a GUI setting for it, I haven't found it.

- - - Updated - - -

Never mind, I found it.

How to change Lock Screen Clock format on Windows 11/10

Turns out there is an option to copy the current user setting to the welcome screen and system accounts.
But it's only accessible through the control panel region settings under the administrative tab.

Using the "new" Region settings control in the start menu, the option to do this is missing. Another case of Windows GUI multiple personalities/schizophrenia. I don't know what UI designers are thinking by having half the settings in one place, and the rest in another.