Here is another bug dealing with the footman demotion.
The 2500 to get past apprentice is, as you mentioned, intentional. The first time a Hearthling levels up, on ANY job, it takes 100 xp. The second time, it takes 400 xp. The third time, it takes 900 xp. (etc.) It doesn’t matter if all of those levels are on the same job or not.