get_the_author_meta( $field, 0 ) returns the administrator $field

Directly on the question – no, the get_the_author_meta() function is not supposed to return you by default the birthday date when you provide 0 for user ID, as it is not a default user meta field. Yet, providing 0 is the same as not providing a user ID at all, hence the function tries to get the author ID from the global $authordata variable.

Looking at the bottom of the code of the function, there is this last line in it, which suggests something (a plugin or the theme) might be messing with you:

    return apply_filters( "get_the_author_{$field}", $value, $user_id, $original_user_id );