My guess is that the shader simply tips the balance there. It's a very simple shader, as you can see by examining it (Unlit - Transparent Colored 1). It barely does anything inside different from the non-clipped one (Unlit - Transparent Colored). It's literally a difference of a few lines of code, as well as the "Offset -1, -1" setting. I suggest playing with the changes between the two shaders if you want to pinpoint what makes it tip over the edge.