I found this to be true as well in my 284 Shehane. When I got to the top end it cleaned up nicely.at low pressure's powder will be dirtier.
I never found it to be dirty . But it is when I shoot it is . LarryDon't get me wrong. If it shoots good when I get to that point...I'll clean all night!! LOL.
It burns clean at max loads in my 240 WBY ; 257 Wby and 7mm STW. Accurate also.
Gary