You guys actually took some buffs up to 22k in that arena, and fighters intercepted??? Must have been before I jumped on...and I missed it...dangit! My squaddies and I have been thinking no one goes over 15k around here.
High altitude fights on AH are rare, but they happen. Though in dogfights one of the participants often just dives to the deck.
The highest altitude I ever got during a regular sortie was 42k - I was first shadowing, then intercepting a huge bomber raid. The bombers did fly at 27-30k, the escorts 35-38k. I chose to stay above them for 20mins while giving updates about the formation on country channel before attacking them.
The highest true dogfight was at about 30k against a P38. We ended up on the deck, but nobody did actually dive, we just lost gradually alt over several minutes.