Keyboard reverting on reboot

hollyB: as far as I know the OOBE choice is stored for the user profile that gets created, while the logon screen reads the system (welcome screen) settings, which are separate. That would explain why the layout looks right through first sign-in and then reverts at the logon screen after a reboot, and it doesn't need a policy that locks the keyboard.

Worth testing on one device: sign in as a user who has the Swedish layout set, then run Copy-UserInternationalSettingsToSystem -WelcomeScreen $true -NewUser $true from an elevated PowerShell (it's in the International module on Windows 11). It copies that user's region and keyboard settings to the logon screen and to new accounts, and users can still change their own layout afterwards.

If that fixes it, you could run the same command as a one-time script after Autopilot. I'd confirm the parameter names on your Windows build first.