Further experimentation revealed it to be the leather, not the dye.
I took scrap from 4 different hides : WC chestnut drum dyed, Hermann Oak, Tandy Royal, and Springfield's Mexico tanned #1 double shoulder. I set them side by side and dyed them one after the other.
Two of the four turned white and two didn't.
Hermann Oak and Wickett & Craig took the dye with no hazing.
The Tandy and the SLC Mexico turned white as fast as the solvent from the dye evaporated.
I think I know why the Eco Flo turned white now, too. I must have used the SLC leather on that belt.
By the way, I tiny amount of neetsfoot, applied once the dye is dry, took away the white. I'll check tomorrow, after the oil fully disperses, to see if the white stays gone.
Jake