Hi. This is not a bug. The decision in question specifically exists so countries that don't have a way to change ideology in their focus tree still can. Japan can turn democratic through the Pre-emptive coup focus and therefore is denied access to this decision (This is what the check_has_focus_tree_to_switch_to_democratic trigger is for).
Thank you for your time and sorry for the inconvenience.
Thank you for your time and sorry for the inconvenience.