It sounds more to me like a defect, which is unusual. I have three hydrofarms hooked up and all run accurately. Either that, or your heat tape is defective and the probe is reading inaccurately because of the way it's taped down...? I'm not sure on that one, because I don't tape the end of my probe, I tape the first part of the cord instead and have the probe exposed but against the heat tape.

Good luck, I hope you get it worked out! This is odd!