Not every rider finds the same horses comfortable. Sometimes there are only a few riders/people who can get the very best out of them.
You read it time and time again about Olympic riders who took a horse on, who no one else could manage and went to the Olympics or other high level events. But they had been destined for the dog meat truck.
I have ridden very expensive horses, but they were mostly racehorses, who had won lots and lots of prize money. A couple were over $500,000. But that did not necessarily make them good to ride, just expensive breeding animals.
I do get what your saying though, you would expect an expensive horse to be a push button ride. Perhaps he has had enough and does not wish to be ridden any more and would prefer retirement. Or perhaps he just requires a certain type of rider to get the best out of him. Older horse does not necessarily mean quieter or schoolmaster.
Some top level horses I have seen at world cup showjumping events seem to be the most difficult animals to deal with. Perhaps that is what it takes to perform at top level, because they always seem to be a bit high strung, especially show jumpers.
*** EDIT - I was thinking about you additional details.
- Dressage judges don't care how rough a horses trot is, they solely mark on the horses ability to execute the movements.
- Showjumping all that matters is the technique and that the rails don't fall.
- Cross-country is again jumping and requires a horse that is bold and fearless.
So in this horse prime, all hyped up on feed and fit as a fiddle, he probably fitted the bill. lol