Powders burning cleanly or dirty is not a yes or no thing. Powders have a burning speed and pressure curve. Each powder will burn more or less cleanly depending on whether you reach its optimal burning pressure with your choosen weight of bullet, and selected load hotness.
Powder X may burn dirty with Bullet Lite, 1/2 dirty with Bullet Heavy and mid-range loads, and really
Clean with Bullet Heavy at maximum load.
The reloader controls the variables of bullet weight and how hot. When the reloader choses light bullets and low hotness, they are creating conditions where "dirty burning" is much more likely.
Don't be quick to blame the powder, until you understand the effect of your other chosen variables.