A bend in the z-axis rod should not cause your hot end to drag through the print and knock it off. It sounds more like an adhesion issue. If you have some masking tape / blue painter's tape lying about, try printing on a layer of that on top of the glass. If you've got some PVA glue, dilute it about 1:1 with water and paint it on the glass (my preferred solution).
Movement in the z-axis is more likely to cause banding in your prints than cause them to be knocked off the glass. Have you noticed any banding in what you've completed so far?
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