Keep in mind that, while pressure may max out at about 8-9” of barrel length, it is still pushing the bullet, and should continue to accelerate the bullet - though at a diminishing rate of acceleration - for a long time.
All of the loads I’ve clocked, hand loads or commercial, showed a higher velocity from my 16” barrel than from my 8” barrel. The difference was smaller with subs, but it was still there.
So if a round clocks at 850 or so out of a short barrel, but then just over half of that out of a longer barrel, then something is drastically wrong.
One possible issue is how close the chrony is to the muzzle. Muzzle gasses can distort how an optical chronograph reads bullet passage, so too close will get you goofy readings. Which leads me to ask what chronograph you’re using, and how far from the muzzle was it when you clocked this load out of each barrel?