I think you have it confused. Marquess missions are for 20 points, taking you from 80 to 100 points. 4 missions of 5 each. The extra missions are for other ranks. Now, if there ever has been something different, I cannot say 100% as I remember some updates at the ranks after its initial release. However, from what I see in the changelog, there have been no recorded changes which seems like it has been like this since start.
Since the Black Death/Transport Immigrants nerf, missions became harder at this point as more cards (double) were needed. Missions never got updated after the nerf to Black Death/Transport Immigrants. Now, that a lot of people changed nationalities, these cards became rare, but nothing uncontrollable as we speak for a common card after all.
When you begin the missions for a new country, you start with 0 loyalty points, whereas with your beginning country, you start at 10. This however, only affects your first promotion as the 2nd promotion always starts with you at 20 loyalty points.
Loyalty points were never additive (cumulative, how to say, they do not add up carrying up from the total from previous rank - instead, they start up from a speicific point each rank). So if you do extra missions at one rank, these were never carried over to the next rank. This is because we offered the extra missions to make it more convenient for current rank and not for the future ones. I believe this is the main source of your confusion here, calling a bug there.
Therefore, I fail to see any bug.