Oxone and Calcium Hardness Test

Has anyone had any problems with the calcium hardness test (Taylor K-2006) after shocking with non-chlorine (potassium monopersulfate) product? Per Taylor's web site, interference on the hardness test could stem from metal ions in the water. I have tested for metals (copper and iron) and none are showing up.

I have NEVER had a problem with the hardness test (typically 200-225 ppm) until I used the oxone based product yesterday. After reading Taylor's "metal ion interference instruction", I added some titrant prior to the buffer and sure enough the test worked and confirmed calcium hardness of 220.

Is it possible that the potassium monopersulfate based product could be the source of this "metal ion interference?"

Thanks in advance for your comments. This might be a question for ChemGeek in the China Shop.