Reef Crystals has done this for years, I used it when I first started my tanks in 2004. Although it was mainly Nitrates in the residue. I had to constantly keep my container clean. RSCP (old formula) did this at the end before they switched too. I've been using SeaChem, while it takes a tad bit longer to dissolve, it has been mixing clean.
I guess I'm keeping my mixing tank out in the open, where I can observe it.