Actually this could all make sense - don't some test strips bleach out at high chlorine levels? And didn't someone at Leslie's say you were at breakpoint chlorination? If so, that would mean you were at shock levels and maybe your strips are bleaching out and showing 0. Did Leslie's give you a number reading for your free chlorine?

A cheap OTO kit will be able to identify roughly where your chlorine level is at so that might be be a simple way to test this theory.

Peter