It's definitely some kind of issue with multiple upgrade paths. I reconstructed what you say; with all Medieval and earlier techs, Gunpowder, Matchlock, all techs on the path to Absolutism (Oil Painting, Humanism, Free Artistry, Political Philosophy, Divine Right, Jurisprudence, Social Contract, Absolutism), Leadership, and Cavalry Tactics, plus access to Horses, you can't build Arquebusier any more, but you can train Grenadiers and Dragoons.
Arquebusier is supposed to upgrade to either Musketman or Dragoon, and Musketman is supposed to upgrade to Rifleman or Grenadier or Mounted Rifleman. What I think should happen is that if you are unable to train Rifleman (which you aren't without Rifling) you should be able to fall back to Musketman. But then you can't train Musketman without Flintlock, so Arquebusier should reappear on your list. Somehow a step is being skipped and the Arquebusier is being hidden by only the Grenadier + Dragoon combination, when it should require Musketman + Grenadier + Dragoon to completely hide the Arquebusier.
If necessary, we can patch this by moving Grenadier up to Absolutism + Flintlock, but I'd like to see if it can be fixed at the DLL level, because I suspect that something like this could come up in another situation involving units with multiple upgrade paths.