So I set my riders' and horses training to cross country and did the training. I assumed I had to only do this once. However, today I discovered all had reset to all disciplines. Why is this happening? Does this mean I have to keep resetting to what I want before I train every time?

When I look at your barn and rider they are currently set to cross country not all disciplines? It shows training in cross country on your horses page. Whereabouts are you seeing it say all disciplines?

I was seeing it under the change disciplines and I mistakenly thought that would stay on the cross country setting too. Newbieness has struck again lol!