Yes 99 DSDP .Cat engine 3000 series Allison , exhaust brake . Occasional harsh downshift .
I've put this down to the transmission shifting down two gears instead of just one .
Now when I crest a summit and slow to my control speed I manually shift the trans down to 5th or 4th depending on what I expect of the downhill .
If you don't manually downshift any time you touch the throttle the trans could shift back to 6th , then double downshift when the exhaust brake re-activates.
If the speed is low enough coming off exhaust brake the trans will only go to 5th , then a single gear downshift on reapplication of the exhaust brake will be much smoother that a double gear down.
BTW: I'm guessing your DS with Cummins , is on a Spartan chassis and your engine is an ICS 8.3L , and has an exhaust brake , not an " engine brake " the term for a; multi stage compression brake.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.