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This is not an HTML <input type="date"> control.
Azure AD B2C Custom Policies render the DOB field using:

<UserInputType>DateTimeDropdown</UserInputType>

This component is rendered by the Azure AD B2C UX framework, not my application UI, so I have no control over the HTML or JavaScript.
The order always remains DD / MM / YYYY, and never changes to MM / DD / YYYY.

So the question is specifically:

Does Azure AD B2C allow customizing the dropdown order for the DateTimeDropdown control, or is it fixed by Microsoft?

I'm not using JSON/XML date formatting in UI — only the built-in B2C date dropdown.