You hit the nail on the head, Dysonman
This is an issue that all current dyson ball uprights have, including the new Cinetic Big Ball. When dumping the bin, a small amount of dust cross-contaminates the clean air path (the tube in the center of the bin), which will then get pulled through the motor and onto the exhaust filters. That could be part of the reason why the Big Ball has 2 HEPA exhaust filters. The cordless models and canister Dysons do NOT have the same issue, because the suction from the motor into the cyclone pack doesn't start at the bottom of the bin where it's dumped, but instead the top, and the dust can't easily enter the clean air path. This is something I thought would have been addressed right after the DC41, but Dyson has released several similar models since, and the issue persists.
I got a very used DC41 a few months ago, and while the pre-motor filter was very clean, the exhaust filter was quite dirty, and with more than just carbon dust. Haven't seen that on a Dyson in a while!
The current Dyson uprights are better than ever in a lot of ways, but over the course of their lifetime, the new Dyson ball uprights will let a lot more dirt into the motor than most sealed vacuums. I hope it doesn't lead to premature motor issues.