Paccar MX13 engine brake, problem?

I recently moved out of a C15 379 and into an MX powered 386. So far I'm happy with it, but the engine brake seems to cut out around 1,200 rpms. Almost like going from stage 3 to 2. Is this common?

 

It feels like it's on stage 3 until around 1,200, then it feels like it kicks down to stage 2 on its own. It's a 2011

 

There's not one with the truck. What's the normal operating range? I always ran my CAT from 11-1,600

 

I'm not much of a high revver lol it has an 18 speed. I usually upshift at 16 and downshift at 12, if I take a full gear. Half gears it's usually 14 and 16. It seems to pull better on the bottom end anyhow. Definitely different than a CAT!

 

Back to this, I've been messing around here and there with it.

When I let off the throttle around 1,500 with the engine brake on, it seems like it works full force and really slows the truck down. After a few seconds, right when the turbo starts to spool, it cuts out. After it cut out, I switched it down to stage 1, then to stage 2 and 3 and there was no difference in engine braking power. Any ideas?