Tom,
I am not experiencing any abrupt transmission down shifting when using the engine brake.
To reiterate what I am experiencing, if the speed of my coach is 64 mph or higher then the engine brake is not so effective as it is when the speed of the coach is at 63 mph or lower.
If I happen to be traveling down a grade at a speed higher than 63 I don't feel much of a speed change even with the engine brake set to high. However, once the speed of the coach reaches 63 mph there is a VERY definite change in the amount of braking at the high setting plus I can hear a change in the sound of the engine.